Green. Far and away. My wife has to stop me when I buy shirts, because I pretty much alway buy the green one. I don't really buy shirts anymore, she buys most of my clothes, so that has helped with variety.
I really like brown too. Probably because it goes so nicely with green.
I've actually been thinking about this not too long ago. My favorite color has always been blue. Recently though I find myself liking purple more and more.
Can there really be one? I mean I love them all. I may favor more "broken" and "mid-deep" colors. Like pastels but deeper. But if I had to name one I'd go black. If black is concidered a color. It's just so goddamn versatile.