It's hard to beat the mpx all around for me but the MP5 will always be king of the hill. There's a bunch of good options these days but some type of delayed blowback would be a must for me.
Delayed, and non-reciprocating.
Don't get me wrong, I love shooting PCCs with suppressed subs (or even better the Banshee in .45), but it is still a rifle and you're walking around with a rifle even if it's pistol caliber. I really like the concealability of the pistols while sacrificing none of the firepower.
The SP9A3 Stribog (or Scorpion and the other true braced pistols) is teeny-tiny without the suppressor and the whale tail collapsed. I can fit the pistol, the suppressor, and six 30 round mags in a very small Grey Ghost Scarab, that looks like it would barely hold a laptop.
Pick a totally non-tactical, non-camo color for added camouflage and you can walk anywhere and no one would ever have any idea. It can be pulled out, charged and ready to go in about 15seconds.
IMHO this is the most enormous amount of firepower you can fit in an entirely unobtrusive and stealth package that would allow you to stroll almost anywhere and no one would have any idea.
I'll grant you that I put this package together for more than a range toy, but it's just as fun and easy to shoot at the PCCs. I wanted the most firepower I could get and fully conceal. Honestly, it's so small that no one would ever suspect it, and far less suspect than a violin or guitar case.