For me Flight Simulator is the best games i ever play, then racing games like Live for speed, Gran Turismo 5, Dirt thats great games too.
I've never had an interest in Runescape, but I can't say I wouldn't like it... I've just never really looked into the game. I personally really enjoy World of Warcraft, which I suppose is strange for a girl.
Nothing beats World of Warcraft! The Worgen race makes the Alliance more interesting now. DOTA and CS are also good (for a change).
Counter-Strike: Source is my favourite online game. I've been playing it for about 3 years now. I even have a complete website and game servers based on this game.