This guy knows what he's talking about. Also the blizz classics (sc:bw, diablo 2, war 3) and Heroes III.
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GTA, and Hitman.
Almost forgot about those. I've never played Fable One, but Fable 2 has an amazing single player, and Fable 3 has a nearly perfect co-op multiplayer/singleplayer. I hope in their future releases they allow you to play with more players online as a group and combine your money if you please. :P
Ill always stick to Half Life 2. I just can't find a game with such a story , gameplay , and graphics that were stunning on early PC's +999 to Valve for that.