My preferences:
1. Stockings
2. Thin nylon socks...ankle socks are great
3. Bare
In most situations I just prefer whatever is most ticklish. However, there's something about the feel of that thin nylon or fabric sliding underneath my fingertips while the soles of my victim wiggle and writhe that just fires me up.
Also, the look of a sexy pair of legs encased in slimming, colored stockings just makes Mr. Winky stand at attention. I just think feet look more attractive in stockings or socks, usually.
PS: Socks are especially attractive right after a sneaker is taken off (even better if I am the one taking off of the bound foot hehe) and the sock is still formed around the shape of her foot...mmmm...