As far as looks goes, the only real requirement is hygiene.
After that, my tastes in males differs from my tastes in females. The body-builder, chiselled-chest look doesn't do it for me, with the noticeable exception of Geoff Tate of Queensryche. I usually prefer femme-bois or pretty boys.
For females, I like redheads, but that's not a requirement. My wife used to dye her brown hair red and end up with this just wonderful auburn shade. Ye gods, but it was sexy. I don't necessarily find waif-thin women attractive. Right now, my wife would classify as a BBW. I guess that's why I find BBW's in general to be attractive now: they remind me of her.