Bf109.
Always was, and always will be.
One of the most challenging planes to fight and survive in IMO. Ofcourse, you can survive forever in a 109 if you play it safely, but the real fun in 109s starts with close-range maneuvering - which is probably the only way a non-vulch, altmonkeying 109 can ever get a kill.
The various 'personality quirks' this plane has makes close-range maneuvering very fun. Unlike most other planes, riding the stalls, chopping throttle, countering torque, getting into firing position, landing hits.. everything is a challenge in this plane.