Assuming we're talking about target shooting, not hunting...
Shotguns. Shooting flying targets beats everything else, when it comes to fun.
.22LR AR, light-caliber Marlin lever guns (.22 and .357/.38), cap-n-ball revolvers, single actions, DA revolvers. All fun.
Scoped .30-06 is even a lot of fun if I'm doing field-shooting practice (popping balloons in open country out past 325 yards off sticks).
Ruger .22 target pistols, one scoped, one not. Both fun.
About the only thing I seldom shoot for just plain fun is a semiauto centerfire pistol.