I don't know about the "most" fun.
I have some that are incredibly fun, though, the ol' Glenfield 60, Norinco Paratrooper SKS, 4" K-frames and a DA H&R 676 .22LR-.22WMR convertible DA revolver.
Out of everything I own, the above are truly a panic to shoot. Aside from the S&Ws, they're all cheaply gotten, too. The Glenfield is amazing. It's gotten to the point where I can shoot the thing (accurately!) so fast that it sounds almost like a machine gun. The 676 is a real sleeper, a true tack-driver, whether in LR or magnum mode.
There is a new addition to the family worth mentioning, an older .44 magnum Redhawk. It's steadily getting to be more fun as I get more and more used to it. A good Pachmayr Decelerator grip helps a bit.