Well...Idk bookwise, but set wise there have been soem pretty sick cards. Jace the Mind Sculptor is probably one of the most powerful cards released, but also one of the easiest to destroy.
Umm, another would be Emrakul, the Aeons Torn from the Eldrazi. Again, easily destroyable since he's a Legendary.
There are some sweet combos you can pull off with those 2 (not in same deck of course...that would never work) that makes them seemingly invinvible.
Basically, Jace is a planeswalker, that if your opponent lets you pull it off, you exile his entire library and put his current hand as his library. Definitely a game winning move if you can pull it off. But since he's a planeswalker all you have to do is summon another one on your side and it destroys both of them.
Emrakul...is probably the hardest to counter considering he costs 15 colerless mana to cast. He is totally worth it though. He's a 15/15 flying, can't be counters, has protection from colored spells. He has anhilator 6, which means whenever he attacks (note, he does not have to do any damage at all) the defending player sacrifices 6 cards,
then the defending player enters the defend phase in which he decides what to defend with if he is to defend. This is the key to the anhilator effect, that it takes place on the stack
before the actual attack.
Then if Emrakul is ever
sent to the graveyard from anywhere (including hand, library, field), you shuffle your graveyard into your library. And as if this card wasn't powerful enough, you get an extra turn after casting him.
Those are 2 of the cards to pay attention to from the newest series. Don't get me wrong, there are other cards that are definitely worth it, such as Sword of Body and Mind, and the other swords (forgot their effects =S).
Karn is the planeswalker in Phyrexia, and his effects are pretty sweet too. A lot of people discredit him, but I don't think that they count for the fact that you can use Karn, to exile himself. Summon another one later, proliferate the crap out of him (proliferate is to copy counters already on a card). Then use his ultimate effect. Restart the game, adn all cards that were exiled are now under your control, on the field, on the first turn.
Insane! Now imagine an exiled Karn. You get Karn on the first turn of the new game! Think of the power!
My name is Brian.