For a sidearm, I’d vote for something along the lines of a sig 250 modular setup. For caliber, a 10mm.
The FBI dumped the 10mm because (in part) it was too much abuse for their guys to take for prolonged periods of practice. Also during extended training the round started beating the gun frames to death and there were some disturbing stoppages involving the slides being locked into full battery and refusing to open. Which I would guess is a BAD thing in the middle of a gunfight.
9mm is too weak. 10mm is too strong. 40 is too expensive among other problems. That makes the 45 ACP for a semi auto in the words of the three bears: Just Riiiiiiight