First of all a few words about Invasion limited in general:
It's almost impossible to play merely 2 colors. This may seem strange to you, but due to all of those multicolor spells you will hardly get enough cards that fit in your 2-colored deck.
There also have been played a lot of not only 3 but 4 or even 5 color decks, and with success.
I myself managed to get a five-color deck running in a tournament the day before yesterday. Of course you won't play an even amount of cards from either color, but play 2 or 3 colors main and splash the best cards of the others, e.g. a green/white/blue deck with a black djinn and a black/blue/red dragon legend…

If you're still in doubt whether to participate in a Invasion limited or not, because you didn't like MB sealed or dislike limited in general, all I can tell you is that I never had that much fun playing limited as I had with Invasion. Playing with and against multicolored decks makes much more fun than the sealed tournaments before did.

One more thing:
Consider that the ratings of the cards listed below are based on their performace in limited, not in constructed. If you want to read about the best constructed cards, check out the new article series of tedman's tales!

Let's start with good cards all colors have in common:

Djinns:
All of them cost 6 manas and get -2/-2 as long as the color they have is the most common color among permanents or is tied for most common.
They are gamebreakers if they have all of their power and combine common abilities of their color with strong basic combat abilities. E.g. the blue one is a 5/6 Flyer.

Apprentices / Masters:
Not all of them are that good, but there are quite a few powerhorses like the blue apprentice and the white master. All of them are worth playing, though, at least if you play the 2 colors they support.

Dragon Legends:
There are 5 of them, all 6/6 flying with a special ability you can activate for 3 many if they deal combat damage to a player. They are potential gamebreakers so if you get one out of your boosters, play it if you can.

Split Cards:
Most of them are great, you have to chose yourself whether they fit in your deck or not.

Duals:
If you get any of the 'Comes into play tapped'-duals and it has the colors of your deck, play it. You mustn't have mana screw…

Cameos:
Same as for the duals, they help you to stabilize your mana base

Let's go on with the best white cards:

Angel of Mercy, 4W, Cr-Angel 3/3, {U} - Flying
When Angel of Mercy comes into play, you gain 3 life.

Flyers are always good and 3/3 for 5 Mana is not a bad deal. The 3 additional life saved me in several games. In fact there aren't many white cards that can help you that much getting back board control as this one, although it might seem to be not so good to some of you. But play it and you'll see…

Atalya, Samite Master, 3WW, Cr-Cleric Legend 2/3, {R} -
X, T: Choose one - Prevent the next X damage that would be dealt to target creature this turn; or you gain X life. Spend only white mana this way.

Ok, 5 Mana for a 2/3 isn't that good, but if you're playing a deck with a lot of plains, this card brings you a big advantage: With its help you can either save your creatures and watch your opponent's ones die or you can get enough life to be invincible, or in some cases both.

Benalish Heralds, 3W, Cr-Rebel 2/4, {U} - 3U, T: Draw a card.

You want never-ending card advantage? You want a blocker that can't be killed that easy by direct damage? You want both abilities in one card?
Ok, there it is: This card is pretty good, if you use it to block and use it to draw a card at the end of your opponents turn. He won't be able to even out this disadvantage.

Benalish Lancer, 2W, Cr-Knight 2/2, {C} - Kicker 2W
If you paid the kicker cost, Benalish Lancer comes into play with two +1/+1 counters on it and has first strike

You will usually play it with kicker and as a 4/4 first strike he's one of only a few white fatties. Very useful for both, attack and defense.

Benalish Trapper, 1W, Cr-Soldier 1/2, {C} - W, T: Tap target creature.

The reprint of master decoy is a strong (and in many cases necessary) card for white decks. To be able to deal with fatties that would most probable kill you (like the dragon legends), you really need this nice guy. I love the Benalish T(r)apper.

Death or Glory, 4W, Sorcery, {R} - Separate all creature cards in your graveyard into two face-up piles. Remove the pile of an opponent's choice from the game and return the other to play.

You are playing against a deck with lot of removal? All your creatures died to his cruel spells? Then use this one to bring half of your creatures back to play and he will hardly be able to remove them once again.

Rampant Elephant, 3W, Cr-Elephant 2/2, {C} - G: Target creature blocks Rampant Elephant this turn if able.

This card may seem weak at first, but have a close look:
You can either finish your opponent off by letting all of his creatures block this one and attack with your whole army or kill those annoying apprentices and other x/1(2) creatures you have to get rid of.

Razorfoot Griffin, 3W, Cr-Griffin 2/2, {C} - Flying, first strike

One of the best white flyers as it is also a pretty good blocker due to having first strike.

Reya Dawnbringer, 6WWW, Cr-Angel Legend 4/6, {R} - Flying At the beginning of your upkeep, you may return target creature card from your graveyard to play.

If you think that you can actually ever cast it, it is a power card. But its high casting cost (3 white mana) makes it almost unplayable in many decks.

Rout, 3WW, Sorcery, {R} - You may play Rout any time you could play an instant if you pay 2 more to play it. Destroy all creatures. They can't be regenerated.

This remake of Wrath of God can help you getting out of a dangerous situation at once and afterward you can easily kill you opponent if you still have one or 2 strong creatures in your hand when playing Rout.

Shackles, 2W, Enchant Creature, {C} - Enchanted creature doesn't untap during its controller's untap step. W: Return Shackles to its owner's hand.

Do you need an explanation for this one? I guess not…
It's a kind of white removal and it's the only white card of this kind…

Onto the blue ones:

Exclude, 2U, Instant, {C} - Counter target creature spell. Draw a card.

A creature counter in limited is good and if it's a cantrip, it is even better. (In fact I lost 2 games because of this card as it countered my dragon legend)

Fact or Fiction, 3U, Instant, {U} - Reveal the top five cards of your library. An opponent separates those cards into two face-up piles. Put one pile into your hand and the other into your graveyard.

As your opponent can't know what you have on your hand, he can't be sure what you're in need of. So perhaps you can get 3 cards you urgently need for a mere of 4 mana. Definitely the best blue none creature spell in limited, perhaps even the best blue card at all.

Shoreline Raider, 2U, Cr-Merfolk 2/2, {C} - Protection from Kavu

A 2/2 for 3 mana with a useless protection ability should be one of the best cards in limited you will ask!?
Yes, it is. There are not only a few but MANY kavus out there waiting to kill you. And this card is your best defense against them.

Stormscape Apprentice, U, Cr-Wizard 1/1, {C} - W, T: Tap target creature. B, T: Target player loses 1 life.

I already mentioned the Apprentices above. I just wanted to tell you that in my opinion this one is the best. Tapping and slowly killing your opponent without letting him a chance to avoid the loss of life in one creature is quite amazing.

Traveller's Cloak, 2U, Enchant Creature, {C} - As Traveller's Cloak comes into play, choose a land type. Enchanted creature has landwalk of the chosen type. When Traveller's Cloak comes into play, draw a card.

One of your creatures becomes permanently unblockable and you draw a new card for only 3 mana. Is this a killer card? Um…

Wash Out, 3U, Sorcery, {U} - Return all permanents of the color of your choice to their owners' hands.

If only it was an instant…
However, the power this hibernation like card, which can be used on all colors is immense. A good finisher and also useful too slow your opponent down…

Black beasts are waiting for you:

Agonizing Demise, 3B, Instant, {C} - Kicker 1R Destroy target nonblack creature. It can't be regenerated. If you paid the kicker cost, Agonizing Demise deals damage equal to that creature's power to the creature's controller.

Ok, this isn't a black beast, but a power card. A instant removal for 4 mana is ok and if you can pay the kicker cost, than it rocks.

Annihilate, 3BB, Instant, {U} - Destroy target nonblack creature. It can't be regenerated. Draw a card.

One more really good removal spell, especially if the extra card gives you another one

Crypt Angel, 4B, Cr-Angel 3/3, {R} - Flying, protection from white When Crypt Angel comes into play, return target blue or red creature card from your graveyard to your hand.

Don't look at this one as a sideboard card! It is one of only a few black flyers and the possibility to return a creature from your graveyard to your hand isn't bad either. And finally, protection from white saves it from being shackled or tapped…

Cursed Flesh, B, Enchant Creature, {C} - Enchanted creature gets -1/-1 and can't be blocked except by artifact creatures and/or black creatures.

Can be used either to kill a apprentice or to make one of your creatures hard to block..

Devouring Strossus, 5BBB, Cr-Horror 9/9, {R} - Flying, trample At the beginning of your upkeep, sacrifice a creature. Sacrifice a creature: Regenerate Devouring Strossus.

Like Reya, Dawnbringer this one is very expensive, but if you manage to cast it, it will almost always blow away your enemy. It only needs 3 attacks to finish him off, even if he still had 20 life. And as it is black, your opponent won't easily able to use removal
to kill it.

Do or Die, 1B, Sorcery, {R} - Separate all creatures target player controls into two face-up piles. Destroy all creatures in the pile of that player's choice. They can't be regenerated.

You can kill half of your opponents creatures for merely 2 mana. You have anymore questions about this? C'mon, you know that it is good!

Duskwalker, B, Cr-Minion 1/1, {C} - Kicker 3B If you paid the kicker cost, Duskwalker comes into play with two +1/+1 counters on it and has "Duskwalker can't be blocked except by artifact creatures and/or black creatures.

As he is a 3/3 creature with fear within one card, he is quite good as a player with no black creatures is about to lose due to it.

Exotic Curse, 2B, Enchant Creature, {C} - Enchanted creature gets -1/-1 for each basic land type among lands you control.

Another card that can easily kill those masters, apprentices and such. In Invasion the enchanted creature should get at least -3/-3

Hypnotic Cloud, 1B, Sorcery, {C} - Kicker 4 Target player discards a card from his or her hand. If you paid the kicker cost, that player discards three cards from his or her hand instead.

Imagine that your opponent had a dragon legend on his hand and was waiting for only one more land to cast it. Imagine he had another power spell on his hand he wanted to cast next turn. Imagine you could let him discard both cards at once…
You will almost always be able to let him discard at least one power card he wasn't able to play yet with this one, so take your chance and remove your opponent's threats before he can even try to cast them.

Ravenous Rats, 1B, Cr-Rat 1/1, {C} - When Ravenous Rats comes into play, target opponent discards a card from his or her hand.

They are finally back, cruel as they have always been. Especially if you manage to get them back from graveyard to play to let your opponent discard once again. The special thing about this card in Invasion sealed is, that your opponent can choose between discard a land and probably getting mana screwed later or discarding a power card.

Reckless Spite, 1BB, Instant, {U} - Destroy two target nonblack creatures. You lose 5 life.

The drawback isn't that bad if you consider that you can remove his best 2 creatures and afterward finishing him off.

Recover, 2B, Sorcery, {C} - Return target creature card from your graveyard to your hand. Draw a card.

Another really good cantrip, a must for every good black deck.

Soul Burn, X2B, Sorcery, {C} - Spend only black and/or red mana on X. Soul Burn deals X damage to target creature or player. You gain life equal to the damage dealt, but not more than the amount of B spent on X, the player's life total before Soul Burn dealt damage, or the creature's toughness.

You can either kill your opponent or his best creature and in addition you get a few life. Really a good deal. It costs only 1 more than drain life did, but the possibility to also spend red mana does more than just even this.