I'm a short-a*** and for me the worst aircraft to get in and out off was by far the unlamented Edgar Percival EP9 - one needs to be virually double-joined and heaven help you if you needed to get out in a hurry.. The Vampire's cockpit is also cramed and untidy ... luckily I'm not too claustrophobic.
I've worked on many types but by nationality British ones were horrible - no space to get in an access hole with a normal wrist as well as a spanner. Here I refer to types such as the Spitire (airframe fuel and hydraulic systems especially) and Vampire (working on a Goblin in situ) French ones such as their helicopters were great and American ones such as C47, AT6 and even the Mustang also nice.