Gears 1 or Gears 2?
While I loved every bit of the Gears 1 campaign I am about 3/4 of the way through Gears 2 and am absolutely blown away. The level design is much better, IMO, and the game seems to "flow" better checkpoint to checkpoint. The only downside is Gears 2 doesn't dump achievements on you like Gears 1 did. Who doesn't love achievements?