The "Best" is a matter of perspective and you'll get as many different answers as there are choices. I've been using Gunkote for thirty years and use it for firearms, RC boats, knives, assorted car parts, etc. Yes, I think it's the best because it fits my requirements for a finish.
I can't speak for your choices #2, #3 and #4 because I have no experience with them. The last I heard, Norrell's was just re-packaged Gunkote.
Gunkote will not rust and is 100% non-reactive to sweat, salt water, common acids, etc. It will burnish to a perfect fit which means it's fine for internals. It's heat rating is extremely high so it's safe for barrels that get smokin' hot. It's the perfect finish as far as I'm concerned but it does require forced heat curing. That can make it user un-freindly.
The biggest problem with some spray on finishes is that the sales person makes it seem so easy. "Just degrease, spray on our finish and let it cure for a month. And, if you order now, we'll double....etc." The most important part of any finish is the surface preparation. Appearance and durabilty are only as good as the initial surface prep.
The bottom line is that some finishes are better than others......for a particular requirement. Gunkote Molly Resin fits all of my requirements. Keep yer powder dry, Mac.
Mac's Shootin' Irons
Tuff-Gun Finishes. The Name Says It All.
http://www.shootiniron.com