Once upon a time I would've had a quick & ready answer. Not so much nowadays.
For revolvers, it used to be .357 Magnum for medium-bore chores ... but .44 Magnum for big-bore needs. I was an avid handloader and fine-tuned loads for various purposes. Nowadays it's .38 Spl (usually +P), and only because I often carry one of my J-frames as a retirement weapon.
For pistols, it used to be .45 ACP ... but then I acclimated myself to 9mm ... and then yet again to .40 S&W (after seeing that it wasn't some passing fad 10 years after its adoption in LE
) ... and nowadays it's ...
whichever.
When I'm done with coffee while browsing the forums, I'm going to belt on my 4040PD and go for a motorcycle ride over to the Coast. Why the 4040PD? Why not? Because it's clean and it shoots rather well for me ... and I just don't feel like picking one of my 9's or .45's (or another of my .40's) today.
Since a box of Ranger 165gr RA40TA was on top of a box of 180gr RA40T when I was loading the 4040PD yesterday (and was therefore quicker to grab), that's what I used to load the mags & pistol. I'd just finished shooting a mix of some 180gr RA40T (both the SXT & T-Series versions), some 165gr RA40TA (previous SXT version), some 180gr HST and even some 155gr STHP earlier this week, all of which provided fine accuracy & handling in fast-paced training drills and a qual, so I'm fine with any of them.
Thinking about it, I suppose that a career in LE, and more than 20 years as a LE firearms instructor, have made the idea of a "favorite caliber" somewhat irrelevant,
for me.
Depends on the day and circumstances ...