Winner: Halo 3
Who could guess that 2007's most open-ended experience would come from a linear, and fairly traditional, shooter? Who could predict that the most exciting potential for user-created content and mods would appear not on a PC, but on a console?
Halo 3's generosity and flexibility is truly amazing. Every game mode can be edited. Every rule can be changed. Every map can be altered. Every weapon can be relocated. If you're bored with Team Slayer or Shotty Snipers, you can create something entirely new like Team Predator or Shotty Swords. If the Spartan Laser is annoying the hell out of you on Valhalla, simply remove the damn thing. If the campaign story seems a tad dull, spice it up with meta-scoring or hardcore Skull effects like "no radar" or "no shield recharge." And don't get us started on the possibilities of Saved Films...
All this potential is just beginning to be tapped. In fact, Halo 3's most memorable moments may still lie ahead of us, as they have yet to be invented.