It doesn't get much more fun (more funner?) than a Model 63 S&W or K-22 and a brick of ammo.
In centerfire, my favorite is 45 Colt because it is just so much fun to hear that big bullet impact on a 16 inch harrow disc at 100 yards. It is such an easy round to reload, too. A 9-ish grain Unique behind a 255 grain swc load is good for plinking, defense, or deer hunting--just a pleasure to shoot. I took my Vaquero out today and shot about two-dozen rounds just before dark.