Call of Duty:
- World at War: great multiplayer, rubbish single-player.
- Modern Warfare: multiplayer not as good as WaW, single-player not as bad as WaW.
- Modern Warfare 2: haven't played it, but I've heard it's pretty much like the first one.
Left 4 Dead:
- Original: great co-op action, *awesome* Versus mode (one team plays as survivors, another as infected), but single-player is lacking. Nothing at all like Resident Evil other than the fact that both have zombies.
- Second: only played the demo, but it sucked with the strenght of a thousand suns.
Dawn of War 2: very fun real-time tactical game. Keep in mind, though: tactical, no "base building" BS here, it's only you and your troops so if that bothers you, get something else.
Diablo Battle Chest: bleh, Diablo II is outdated as hell, not just in graphics but in gameplay as well. If you really want a dungeon crawler, go get Torchlight, it'll frustrate you far less.
Rogue Warrior: haven't played it, though Metacritic says it has an average score of 26 so buyer beware.
Star Wars Force Unleashed: haven't played it.
Soul Calibur: Broken Destiny: never knew there was a SC for PCs, gonna have to check it out.
Fallout 3: great RPG, like Oblivion but better, but the deal with the DLCs is kind of annoying (one of the DLCs adds the ability to continue playing after the final storyline mission, don't have it and it's instant "game over" for you).
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Now, what would I recommend? if you have people to play L4D with, that easily, otherwise... dunno, if you're OK with the DLC thing, Fallout 3 is a great choice, and so is DoW2 if you aren't biased against tactical games. The CoDs are pretty fun for multiplayer, but they're generally overpriced as hell and, if you're spending that much money on a FPS for its multiplayer you're much better off with Battlefield 2.