[ShadowMarth]

Starcraft: Brood War. It's super cheap lately, has an enormous amount of maps, mods, and ranked players, and no subscription fee for battle.net. Not to mention even for the massive amount of mods there's no need to wade through all the patches. Automatic updates, and whenever you enter a game with a map you havn't played you automatically d/l it quickly. It's one of the best RTS games ever made.