Although I like slouch socks, they are kind of too thick to tickle all but the most ticklish of people in. (Ticklish in slouch socks would be one way to really measure ticklishness in anyone!) Usually I prefer socks that come up over the ankle, but stay below the knee. The cute socks with frills and lace are also nice. As for color: pink, white, red. And if there are cute designs like hearts, stripes, and polka dots, all the better. I absolutely hate the "no-shows" or any of those that don't go over the ankle or at least to it. Might as well be barefoot at that point. (And I would rather see barefoot than "no-show" socks.)
It's all about cuteness, and if a punishment scene - anything that would add to humiliation factor for the ticklee.