Most fun - M3/2/4, Plaubel Makina, Mamiya 6/7, Rolleiflex 2.8/3.5, the various LF cameras I've used have all been fun, Yashica T4/5.
Least fun - F100, F6, EOS 3, EOS 1N, EOS 1V, Pentax 6x7/67/67II, RZ67.
My least fun cameras are mostly cameras that are really good but they are just boring. They are great at being work cameras when you just need to get $h!t done but otherwise no fun. And the Pentax 67 is thrown in there just because of the weight. It's a fantastic camera and I wouldn't mind owning one but I hate lugging them around. Same with the RZ except I don't have any desire to own one ever again.