Somewhere over by MK--GF, Poly, CR, WL, or FW. My DS and I shared an hour (special rate of $32) and it's one of our favorite memories of the October 2002 trip.
Depends on where you are when you decide you want to rent one. If you're in Epcot, just walk ove rthrough the back entrance to the YC and rent there. If you are in the MK area, the Poly, GF, and CR all have them. The rental price is the same everywhere, so it's just where the most convenient place is for you to get to.