I would say that light playful tickling can happen with socks but if the lee wants more intense and deliberate tickling, bare skin is best. Professional tickle torture is almost always conducted without socks on.
I would say bare feet because it’s easier to use tools and nails. Socks just get in the way and every lee I’ve had said bare feet was the worst (or best?) experiences they’ve had. I think it’s also easier to tickle bare and the skin is easier to touch in different ways with different tools and methods.
Have to agree with everyone and say barefeet. lol However I also like if the lee starts out with socks, tickle them to test their ticklishness and then get to the real deal by taking them off and tickling barefeet. hehe