Highest:
SIG P226, 9mm, factory mag 15rd, but I always load the MecGar 17rd flush fit first.
Lowest:
I used to love DA derringers, and have owned several, in .38 spl and .357 mag by American Derringer and in .22 WMR by High Standard. But in the "LCP era" they seem clunky and limited at best. They're all gone.
My lowest cap handguns now are J-frames, 5 shot; a 442 and a model 36.