Any game that is moddable and online is probably the real answer, so that is bf2 and the hl2 engine to me.
Both can be modded and have voip and 64 players, I dont know of any other engine that can say that.
CS, etc are good but I prefer the vehicle and wider spaces of bf2 and its mods.
Disappointing in a way but they wont be usurped by any future game afaik