A few favorites come to mind as I read the question. Can't narrow it down to one, as there are different tasks for different airplanes I have flown. There may be some cross-categorizing, but that's aviation...
Airliner: DC-8-72
Turboprop: L-382G
Twin: Twin Otter (learned in a Twin Comanche, so that has good memories)
SE XC: C-210 (Comanche's are good, but I like flying in the shade)
Utility: C-185, Beaver, C-206, Super Cub
Trainer: I learned in 150/152's, and the basics were conveyed properly.
ME trainer: Piper Aztec (a solid platform)
Steam gauge rules. Glass, besides my iPad, leaves me cold.