From best to worst:
1) UT3
Um, he didn't mention UT3 you know.
Anyway, I've only played two of them (Halo 3 and Gears of War) so I can't really comment on the others. But of those two:
1)single player mode - Halo 3.
4 Player online and lan co-op primarily. Plus vehicles! And lots of guns!
2)multiplayer - Halo 3
Larger community, better online matchmaking, more game modes. Twice as many players in a match. Vehicles!
3)graphics - Gears of War
Halo 3 looks quite pretty, but it simply can't match the unprecedented detail in Gears of War.
4)overall who do you choose - Halo 3.
Gears' single-player is even shorter than Halo 3s, and offers no room for exploration. Online co-op, spawn-editor Forge and greater range of game modes, seal the deal for me.