As far as content goes, I'd say how good of a performer she is can be important - how good is she at dirty talk, how does she moan, etc. But in terms of appearance, #1 is big, perky tits with small areolae, pointy nipples, and minimal cleavage gap. Next would be a nice hourglass figure - not too long of a torso, a tiny waist (ideally ~22-23"), and nice hips. Shapely but slender legs, a great ass (not flat but not too big), smooth pale skin (ideally with no tattoos), long neck, and weirdly enough, good posture are all turn-ons. I'm not particular about hair color, but hair style can be important. Eyes, any color can look good, but I am partial to green, blue, and grey.