Halo 3 is probably the best all around because tons of people still play multiplayer.
Vegas 2 is too short, beat it in a day, and gears is fun, but there are too many hardcore people on multiplayer.
The best offline/online game is the Orange Box. 5 games. Half Life 2, episode 1 and 2, Portal, and TF Classic. The 3 Half Life games will entertain you for many gameplay hours, Portal is an awesome game, but short, and once you finish all of those you can play TF online. Best package ever...the best part is, now its only 20 bucks!