= Thunderbolt. Quick, simple removal that kills just about anything short of a dragon. Honorable mentions go to quintessence and mindgate.
= Eagle's Eye. I can't tell you how many times this one card has wtfpwned my opponent in tournament play.
= Improved Steal. Vampire Dagger/stiletto gets a close 2nd.
= Arsenic. I've never been a huge fan of the death element, but this is a nice card in that it keeps damaging the opponent even if they destroy it, and it's one of the few death cards I've used a lot.
= Pulverizer. Repeatable permanent removal kicks arse. Graboid and quicksand are definitely close seconds, though.
= Supernova. Pretty hard to pick anything else. Purple Nymph has won me a lot of tournament games on its own, as has Discord, and they'd make good runners-up. I also like antimatter.
= Explosion. Dirt cheap permanent removal, even after the +1 cost mod. Phoenix gets runner-up.
= Amber Nymph, easily. This card can lock the game if unanswered, especially against rainbows. Otyugh and Gravity Shield are runners-up.
= Giant Frog. I'm not a fan of the life element, even though that's what I started with. Giant Frog was a key card in the ai3 grinding I did my first week.
= Sanctuary. I love this card, especially after the SoG nerf. It heals and it counters several of the most overpowered cards in the game.
= Electrum Hourglass. Card advantage is pure gold in any CCG, and this card gives it. Golden Nymph is awesome as well, but more expensive. Eternity or GotP are pretty good, too, but not in that same class.
= Steam Machine. 15hp makes this thing pretty much invincible, and it gets pretty darned big. Permafrost Shield gets runner-up.
= Shard of Sacrifice. Probably the only card more overpowered than Gravity Nymph. It's wrecked the entire metagame, and probably only supernova can also claim that.