During all these years of playing Hearthstone, I have won countless games thanks to Flesheating Ghoul. Card never shined in constructed, but can be a real monster (literally :P) in arena, sealing games pretty fast if left unchecked by your opponent (which for some reason, usually happens). That's why it is my all-time favorite arena card. :)
Death Stalker Rexxar. I really like the card and also it was fun to just be overpowered.
Eh, all hero cards are rather absurd - much like I'm not going to start talking up Illidari Inquisitor because it's obviously a nuts arena card. Horrendous Growth and Deeprun Engineer really feel like they're stupid, though, currently. Engineer will usually get you a claw machine for value.