Though question. My definition of a "fun" gun would have to be one that is relatively cheep to shoot, but still makes a big "impression". I'll let each shooter define impression. A fun gun cannot be a workhorse or a collectors item. It must be a shhoter.
My personal list of fun guns would include-M1 Garand, M1 Carbine, M-N Mod 44, Trapdoor Springfield, Original mod. 92 in .44 WCF, Mini-14, S&W K38 Masterpiece, Ruger Single-Six .32 H&R...
Granted some of these are collectibles, but they are shooters first in my book. As far as my favorite, it depends on my mood. If I want to play cowboy, I'll pick one. But if I want to pretend to kill germans, Japanese, or Indians, I'll pick accordingly.
Wyman