I agree with the others that say the Time Expander on Doctor Who.... although I did get good at getting it in and out when I had to fix mine. Must have been in and out of the machine 50 times. This was before all the guides to fix it existed!!
The top part of Earthshaker with the split is challenging as well, as it the top ramps on Taxi. The playfields really need to come out of system 11's to work on the top stuff, inside the cabinet it's too difficult to bend over into there.