Assuming that we're being happily superficial and talking only about physical preferences and not the many other qualities that go into making a guy hot...
I tend to prefer Caucasian men. I mean that inclusively, incorporating Europeans and men from West and South Asia and North Africa. Within this group, I like men who are on the dark side and hairy, so I'll go first for Mediterranean types and guys from the Middle East and South Asia.
(Iranians seem to be just about the hairiest men on the planet. That's a GOOD thing!

)
On the other hand, I've seen many very sexy men from more northerly regions, so it's not an absolute thing.
Despite liking hairy types, I also like East Asian men if they are reasonably tall and in great shape; this is most often true of men from northeast Asia (China, Japan, Korea) and Asian-Americans. It's a shame you hardly ever see these guys in porn.
Men from Southeast Asia (Thailand, etc.) tend to be too slightly built for my taste.
Men from sub-Saharan Africa I'm not usually attracted to unless they have really spectacular bodies.
Mixed race men, especially African/European, can be fantastically sexy, which probably explains why Brazilian men are so damn hot.