Even though I carry a .45 now, it's more because I like the gun itself as a whole, not necessarily the calibre. I'd say the "best" cal for shooting targets is probably .22 LR, it's so versatile, cheap, and available. For defence it's gotta be 9x19mm as ballistics experts, including those in the FBI who ended up ditching 10mm and .40 (and not even running with .45 much), have concluded that with modern defensive ammunition, there's little to no difference between a wound between any of the major handgun cals. A .45 HST is gonna look almost identical to a 9mm Gold Dot. If that's the case, may as well do what all the world's LE and .mil do and adopt 9mm for the increased capacity and lower recoil and cost.
Now for my favourite, that's a different story... My fav target calibres ever are .45 Colt and .44 Special. Back when I was reloading I'd load up as much of these as I had components for, then go to the range and shoot til my hand was sore. Then I'd be back at home reloading the same casings in preparation for the next range trip. Something about that thump and deep, bassy boom of those calibres is super satisfying. Same deal with simply loading those fat cartridges into my Uberti Bisley or S&W 69. Can't get enough of big bore revolvers.