most reliable 45
Basically, if you're going to buy a NIB gun from a major manufacturer, that's a classic example of a crap shoot. You won't know if your gun is a keeper until you go through breaking it in. All manufacturers, without exception, have "lemons" from the factory on their mass produced guns. You have no guarantee that all the parts are 100%, nor that a skilled gunsmith put the gun together. So if you're not up to the task of gunsmithing, nor want to soup the gun up, (as I assume you're not, by the "out of the box" statement) go to a gunshop that will take care of you if the gun isn't 100%. Beware, some manufacturers can take many many months to get your gun back to you if it needs work and you have to send it to them. Either way, take the gun to the range right when you get it and make sure it is 100%.
The only way to buy the "most reliable" .45 is to have someone sell you one of their .45s that has a lot of ammo through it with no malfunctions. And most people don't sell those guns, they sell the finicky ones.
Just make sure to take your gun apart and clean it before you use it. I've know some people to complain that their gun jams, and then I find out they've never cleaned and oiled it yet. Guns often have stuff on them to keep them from rusting that needs to be removed for reliable operation straight from the factory. And a little lube can go a long way towards reliablity.
If you're considering a 1911-A1, (which is a good choice) get a full-size barrel. I've never had any problems with my SA Mil-spec, and it conceals well in a good IWB.
I've heard nightmares from the micro compact 1911 owners, though.
Revolvers are the usual sure bet for reliability, but I'd rather have a .357 mag than a .45 acp revolver, for the most part. I'd say a 1911-A1 with a .357 mag as a backup gun (BUG) is a safe bet. Just shoot a thousand or so rounds through the 1911 before you bet your life on it. Don't use cheap mags, either. With Chip McCormick PowerMags, I know mine will go pop-pop-pop-pop-pop-pop-pop-pop-pop, every time. I've never had any ammo feed issues, but then again I use S&B FMJ, Corbon Powerball, and Winchester JHP/FMJ ammo in my 1911-A1.. quality ammunition. I also keep my feed ramp clean. If your gun isn't a lemon, reliability is in attention to the details.. like keeping your wrist stiff, mags, and ammo choice.
Good luck, and have fun gun shopping!