9mm - Tepid performance so 357mag, 40s&w, 45acp, and 10mm are better choices.
In my safe, I have .357 Mag, .40 S&W, .45 Auto and 10mm, and more but I simply don't have a better choice for dropping steel plates than my 17-rd 9mm.
It's accurate. It's outrageously cheap to handload and it doesn't matter if I pick up my brass. It drops every plate that manage to hit with it. It runs flawlessly and doesn't choke or stop. It doesn't muzzle-flip much which help me put the sights on the next target smoothly.
If there's a better setup for fun & enjoyment at a wide range with many steel plates, I'm not sure what it is.