If 1911 platform... .45 ACP (best reliability from that platform)
If Glock Platform... .40 S&W (largest caliber available in std frame size)
If "mini" Platform... 9mm (more managable from small frames)
You didn't ask but if revolver platform... 38 spl (easy to handle and proven performance...not .357 mag due to EXCESSIVE blast and flash)
Just my $0.02
+1. . .I do shoot some 357 mag in my snubbie though, but usually CCW 38 spcl +P.
To add, Wilson seems to think the smaller 1911's are best done in 9mm or 38 Super for reliability. . .Who am I to argue.
My "mini" is a larger P40 in 40 or a smaller P32 in 32 ACP. No 9's for me yet.