Tie between:

Energy Sword - Halo 3
The Rage power, that turns enemies against other enemies. - Bioshock

I love having control over my enemies, watching them attack each other helplessly, it's why I love RTS games so much, and why I'm so pumped for Halo Wars.

I just posted on a topic similar to this, the poster was asking what xbox to get.

The 60GB pro is your best bet, by far. My friend got the Arcade one, whilest I got the Pro one, and he regrets it deeply, simply because he can't do alot of things without a HDD. Oh, and the Pro xbox looks better.

Mario Galaxy has the best singleplayer, whilest SSBB has the best multiplayer, and I don't buy singleplayer-only games, only rent. And I only buy good multiplayer games, so I wouldn't buy Galaxy ever, but SSBB is a keeper, so I'd say SSBB because of it's multiplayer.