I've always heard (and experienced) that Al-Aroos saloon was the best in town. They're located on Al Sadd Street, behind the Best Fish restaurant (kind of in the breeze way of a building), about 3-400 m on your right side of Al Sadd. They only do henna (and threading I think) -- so they're not a full-service salon, but maybe that's why folks think they're the best.