StarCraft bud. Huge community still going strong, although the skill level is through the roof. A 'noob' player could absolutely destroy someone who has been playing for years but hasn't taken the game seriously. If you want to get into Starcraft, make sure you are willing to dedicate yourself partially to the game. Also don't play on bnet, get iccup [http://www.iccup.com/].
WarCraft 3 also has a bit of a community going for it, but other RTS's I'm not sure. I mostly play LAN games with my friends.