I have 4 legendaries that I consider extremely powerful:
Plants:
Bananasaurus: fairly cheap at 4 sun, to get a 3/3 that attacks twice. The fact that it grows every turn and even more if you play something that draws makes it extremely powerful and difficult to stop if you play it right.
Doubled Mint: also very cheap at 2 sun. The trick is to play it early when if the zombies "tap out" (use all their brains) and then the next turn buff it. Once it gets past a certain threshold, only direct kills will remove it.
The fact that both of these plants are cheap to play make them immensely powerful if played in the right deck, at the right time.
Zombies:
Trickster: This guy is usually played for 4-6 brains because you will put him in a deck with lots of tricks. You basically get a 6/6 and either 6 damage to the face or destroy one of their plants, quite powerful for it's cost.
Barrel of Dreadbeards: This guy is hard to play effectively sometimes, but can be quite brutal if played turn two and decimates out their board. Also a great combo with Fireworks Zombie and Newspaper Zombie.