I think .357mag is the most versatile chambering and it tends to be what I suggest for a first or only revolver for that reason. It can be downloaded with inexpensive and light shooting .38spl target ammo, loaded up with good defensive .38+P, defensive .357mag, or the barnburning hunting magnum rounds. I think .38spl/.357mag in a 2 1/2-4" K-frame or L-frame sized revolver (especially the K-frame size) has to be the most versatile combination of platform and caliber around.
Still, that wasn't the question, it was favorite. It used to be .357mag. All the versatility issues above, and when shooting full-house magnums it is a blast to shoot. However, now it is .41mag. Not as versatile, but so much more fun. It isn't punishing like the .44mag, it is much more powerful than .357mag, and it is a ton of fun.