Only 1 vote for a caliber, huh?
Okay, I went .38 Spl, since I've opted for a J-frame snub more often than not. Once I got into Airweight J's (instead of steel), I could often be found pocketing (holstered) a J-frame snub after long hours of wearing the issued weapon (which ran the gamut from 9, to .40 & .45 ACP).
Now I own more than half a dozen J's.
Sure, depending on my planned circumstances for any particular day or night's activities (and areas involved), I may opt for a belt gun. If so, I'll often reach for one of my many 9's, .40's or .45's, but more often than not I prefer something smaller & lighter on my own time ... and especially now that I'm retired.
I've even gone to pocketing a LCP .380 at times, especially if I'm wearing jeans with front pockets that are too short & tight to accommodate one of my J's. Just picked up a new stainless LCP to supplement my blued model.
I remember as a young cop thinking that the only "worthwhile" calibers to carry on-duty were either a .357 Magnum or .44 Magnum, and off-duty either .357 Magnum, .45 ACP or .44 Magnum. I grudgingly accepted 9mm when they took my issued .357 away.
However, time, training and experience often changes things, including perspectives.
I carried issued 9's, .40's & .45's over the years, and added several 9 & .40 to personally-owned calibers.
Since I still keep my hand in things as a firearms instructor and armorer, though, I still get in a decent amount of range time for training, drills & quals. I'm one of those guys who thinks it's enjoyable and relaxing to shoot 5-shot snubs in DA/DAO configuration, even using the assortment of +P I use in my +P capable snubs. I'll often take them out to 45-50yds to ring steel, too.
Since 9, .40 & .45 are issued service calibers, I've no trouble getting in enough range time with pistols which use them.
I still prefer the .38 & .380 more often than not, though. I spent enough years having to wear a larger belt gun all day/night.