I like my Glock 17. Being LE, you can get "intended capacity" magazines for a decent price. All Glocks are the same, so I'd say a 21 would be good, if 45 is what you want.
If you want traditional double-action(unlike Glocks safe-action) or double-action/single action, and having an option for a traditional safety and/or decocker, the H&K USP's(9mm or 45) would be the way to go. Though, I don't have one yet so I'm no expert. I'm looking at getting a USP45, but that's after I've collected a few other guns.
I don't like having a safety on the slide(might pull on the slide and engage the safety during high-stress situation, not that this has ever happened, but it could), so the Beretta 92's are out, unless you want a double-action-only 9mm.
S&W is not an even an option.
So to summarize, a Glock or H&K USP would be my choice. I know compact Glocks can accept full size Glock magazines, so if you are wanting a back-up gun, Glock is the way to go. I don't know if compact H&K's do this, besides they don't make their compacts as small a Glock sub-compact(are you even allowed this small a back-up if a back-up at all?).