Battlefield Bad Company 2, for online, and Halo: Combat Evolved for singleplayer.
I love both of these games and shall never NOT play them. I haven't played BF:BC2 in ages, due to my ps3's Internet being screwed up since December, though I have had many dreams of playing it again for it's marvellous multiplayer. *sob*
As for Halo: CE, I play it ALL THE TIME. Great game. The only halo game better than H:CE is Halo: Reach.
And, again, I don't have an Xbox so playing H:R is almost imposable without going to a friends house.