Another vote for KelTec Sub2000. It's like a 10/22 on steroids. Everyone who shoots it (except lefties who get powder residue from the ejection port) gets a big grin.
A neighbor has a P22 with a Gemtech Outback II can. The only thing more fun than a suppressed .22LR might be a full auto suppressed .22. Some of the high end Airsoft guns actually get pretty close. The Drozd full auto BB gun would be great if the CO2 cartridges didn't ice up. Another acquaintance let me shoot a practice round through an M203 40mm grenade launcher. That wasn't as much fun as a suppressed .22, but chalk is a poor substitute for a satisfying detonation on target.
I don't remember any gun that wasn't fun, except maybe one that was broken, out of ammo, rusted in storage, cut up or de-milled, plugged with a squib load, or used as a symbol by some exploitive politician or bed wetter.
Never had a bad day fishing either. Most guns fun.