The Bell 212 (430) Series and the some Jet Rangers can be tough. The Sikorsky S-58 has to be the worse I have done. Both piston and turbine versions. The low-lead nozzle doesn't seem to fit nor does the J-nozzle, so all three tanks have to be filled very slowly... The worst I can think of single point wise has to be the AH-64 Apache, especially when there is ordinance on the wing. It takes a while to get everything lined up and when there is ordinance you cant run the hose over the pylons to line up the nozzle any easier.
Easiest to fuel chopper wise has to be anything else single point. The Erikson Air Crane as well as the Chinook have probably been the easiest in my experience. Over the wing style would have to go to the BO-105 or the BK-117. Robinson's are also pretty easy and quick to fill up.