It used to be The Handler, because it sounds the most musey to me - minor-based, and I imagined Micro Cuts spawning from it.
I initially didn't like Reapers. I thought the chorus was funny and ruined the build-up it got. However, now I love it, I love the fast pace, production. The drums are so cool. I didn't listen for them in the beginning, but they are cool as shit. I often put this on when I'm about to get to work, when I need to get pumped up for some reason. I also imagine mastering this on Guitar Hero with a friend who plays the bass track.
Right now it's my favourite Muse song across the board.