Attitude is the number 1 thing for me, along with her reputation and hygiene. Then services offered and the price of those services. Then location, availability, and ease of getting in contact with her and arranging something with her.
Looks come after all of these things, but the girl does have to be attractive to me otherwise I wouldn't want to book her in the first place. Although by tastes are quite wide, size 6-14, 18-45 years old, white/black/asian, British/EE/Oriental, blonde/brunette/redhead, big/small ass/tits, I don't really mind all that much as long as she is reasonably attractive. The more attractive she is the better though, obviously.