I love games heavy in roleplay. I'm not so enraptured in roleplaying where I shun game mechanics to play out how things happen. I really enjoy D&D 3.5 and 4th edition. My favorite GMs would have a really good grasp of the flavor of whatever setting they use. Also, a good GM will know where the system breaks down and has his own set of good house rules that he can present up front when starting a game that addresses that (list of restricted classes, home-brew system to alleviate stacking of class bonuses, etc).
I have been a player and a GM for many games since the early '90s. AD&D 2nd Ed, D&D 3.5 & 4, Shadowrun 3 and 4, Serenity, Star Wars D6, and many others. I like to play every week, but have been making do with DMing a 4th ed D&D game once a month.