According to some lesbian friends the vacu-lock 2000 harness and strap on is the best...there are many on the market that don't hang properly and are difficult to direct. (ya know...direct?)
Also, if you are purchasing the strap on for a woman, check out the ones that are like a double ended dong, one smaller curved end goes inside the woman and the rest sticks out of her like a penis. The part that sits in side the woman creates pressure on the G-spot. I think there is one with a vibrating bullet as well....