Someone familiar. I'd much rather be tickled by someone I was close with. At least then I could talk to them about doing stuff I liked better. Communication is good, if hard. Also, they already seem to know what I like, which is a rare skill.
Would you rather find yourself in a relationship with two separate people, both of which indulged your tickle fetish, and you now had to choose one... or be with a perfect partner, with the exception that they wanted nothing to do with tickling, either tickling you or being tickled by you?