Well, first and foremost, someone that is single. I can't tell you how many women I have met lately that I am compatible with, and interest me, that aren't single. I can't tell you how frustrating that is.
Other qualities:
Physical appearance: I am not super picky about hair color, eye color, etc., but there are certain body types I am more attracted to than others.
Personality: Sweet, smart, honest, straightforward, understanding (this is a big one), not naïve, and NO major personality problems (narcissists, histrionics, borderlines). I have met a bunch of women with personality disorders lately, and I can't handle being around them too long.
Compatibility: We have to be on the same page with our interests, lifestyles, and values.