Hands down favorite is .38 Special in all it various manifestations. Has
been a favorite for decades. I like 148 grain wadcutters. There is no better shooting experience than shooting clover lead groups with .38 wadcutters on a nice Sunday afternoon. Enjoy 158 grain RNL (even more accurate than WC). Like hot loaded +P 158 grain SWC. I shoot .38s all the time, reload for it, cast bullets for it, etc. Have multiple 5 gallon buckets of empty .38 brass. I tend to think some flavor of .38 will do about anything we could reasonaly expect a handgun to do.
Second favorite is probably .22 LR, for all the reasons the previous posters mentioned.
My third favorite is fairly new to me. .30 Luger (7,65 Luger). I got an old 1929 model Swiss Luger that is stupid accurate with this round. Flat shooting, low recoil, but really clangs the steel plates. Different. Bottleneck. I'm infatuated, at least for now. Maybe the thrill will fade.
Honorable mentions include .44 Special and .32 S&W Long.
Shot a lot of .357. I respect the round, but would not say it is actually a favorite. I always tend to find it too much of a good thing, and ease back to hot .38 Special.
Shot a ton of .45 ACP. Again, I respect it. But never really got to be best friends with it.