Probably half life/HL2/Deathmatch. I can play them again and again, and considering the amount of mods, the possibilities are endless. On the other end of the spectrum though, would be left 4 dead, by the same makers. You play each campaign once, and you realize that you sort of regret spending 40 dollars on it.
God damn it why are there only 4 campaigns that last only 39 minutes?
Though Far Cry 2 is also replayable. it takes a long time to beat but there are many choices to explore and the maps are huge and influenced by the weather. It's very nice.