It depends. If you want to find a ton of players for Team Fortress 2 along with updates AND a lot of neat mods for Half-Life 2 and Portal, then get the Box. If you want a shooter that will give you deja'vu (hey! power armor!) coupled with an overwhelmingly mediocre expansion, then get the Crysis pack.
I'm an RTS and RPG man, so my real suggestion would be Empire: Total War (little buggy though), Dawn of War II (steam), Demigod--hell, take a blast from the past with Stronghold. The Witcher is also amazing.
If you're a shooter diehard though, get Farcry 2, Bioshock, or ArmaII.