Yeah, sometimes it's tough to choose just one fun gun. Just as in golf there is a best club for a particular situation, so the fun gun choice invariably involves what particular shooting is preferred.
I also really like several different guns, but my favorite is the Ruger Single-Six .22/.22 Magnum because it best matches my favorite shooting situation: inexpensive plinking at short ranges.
In my suburban environment that means home made targets at an affordable indoor firing range. Tool + Situation - Cost = Fun
Now don't go having too much fun with that equation.
