While the obvious answer is none (I follow cheapshooters rule) if I stick within the rules and also ignore the obvious of picking one of the .22's (they are low hanging fruit as there are so many of them, especially the 6" NEOS since I have 2 NEOS and i like the 4.5" one better) then the question is a challenging one.
I'd have to pick the KP345. It's not that I don't like it, I do ... I like it a lot. And even though it's my only .45auto, I still like all of the 9mm's so much better. Even with 4 9's the .45 has to go as I really like each one of the 9mm a lot and I'm just fond of the .45.
The pecking order is quite simply:
92FS (way out in front of the rest)
Cougar 8000F (tied with the 5906)
Oh yea, and the Bersa Thunder .380 - It would be a tough call between the Bersa (my only small carry gun) and the Ruger (my only .45)
597 VTR, because there's so many cans and so little time!