I have to say the trust factor has to be the #1 most important thing when it comes to finding someone to be a lee. Someone that trusts you not to go too far and, if you so decide to allow them revenge, someone that you can trust not to just tie you up and act like they're going to tickle you, but they end up robbing you as you are lying there, struggling to break free.
The person's ticklishness comes next. A lot of times back when I was still single, I often found my ex gf's claim to be really ticklish but when it comes time for them to get tickled, they only giggle and squirm, no real laughter. So, ticklish level comes second. THEN the beauty of the lee would come. I'm not saying that I used to tickle just ANYBODY, I did have some standards, but really deep down inside didn't care about their looks...