RPG and strategy games are the best genres, in my opinion...

If the first one is about romance, the opportunity to feel like a hero, and to go on "real" adventures that you've always dreamed of (mainly Neverwinter Nights, Baldur's Gate, etc.), then the second one allows you to demonstrate your intelligence, your quick wit, and your logic in devising tactics and other military tricks, and also to feel like a commander.

I always switch between the two... after playing an RPG, I start craving strategy games, and vice versa... that's why I mostly have games of these genres on my computer.

Sometimes I want a quick game... for such cases, there are racing games (FlatOut 2, because I like racing games where you can break, smash, and crash everything), sports games (I used to like hockey, but the others aren't very appealing), and shooters (Quake, etc.)...

P.S. In general, I don't really like shooters, sports, or racing games... unless it's online =)... although there are some interesting ones even without online play, but there aren't many.