halo 3 is a decent singleplayer, if not a bit short, but if you're going for xbox live, its the way to go
Gears is a neat singleplayer, it'll have you enjoyed for a while, but the multiplayer is awfully disapointing, with host advantage, lag issues and only few players online nowadays
the orange box is one of the most acomplishing packages available, and its sure to have you enjoyed if you love Half Life, though i dont know about TF, people complain about it having lag, but i guess Portal, a very short plus game, makes up for that
ADDITION TO THE LIST
Call of Duty 4, even more players online than halo 3 has, the multiplayer is simply increddible, it has some really neat maps, custom class creator, and its a complete package of entertainment, only problem is that the server connection isnt all too good, and sometimes, you have to wait a really long time to come in to a game thanks to that, but there isnt much lag