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.   
