My ultimate favorite would have to be inverted Y (wrists tied together above head and feet tied apart) closely followed by something like this:
😀 😀 😀
Ok, this would come in a very close second to a spread eagle position! Being that I enjoy tickling all over my body, this would work best for me, esp. being that my underarms are my most ticklish part of my body. Plus, I really like to do the whole eye contact thing while being tickled.