Remember the black topcoating on the nitrocarborizing finishing like the old Tennifer, the replacement GLOCK calls "nitration," Melonite, and the dozen similar finishes is just that. A topcoat. The surface of the metal itself is treated and changed. A black finish is then applied over that. It's why people can polish their glock and M&P slides and not have any rust issues. It's not exactly a bare steel.
The black that glock was using on it's post tennifer, but pre-gen 5 guns wore off faster than it used to on the tennifer.
The most durable non-metallic finish is Cerakote. You just have to find a good applicator who knows not to put it on too thick on the inside of the slide, the rails on the frame, or the barrel. I've seen a 1911 that needed quite a bit of work just to get it to be able to hand cycle without inducing muscle failure. I think he took a case or so before it would cycle without issues.
Black chrome is pretty cool looking. Can be had in matte or polished. Most of what I've read is that it's not as durable as hard chrome. Scratches will show the bright silver undertone. It's supposed to be as weatherproof as hard chrome though, but weather and wear resistance are different.
Robar is great. I'd love to have a couple guns sent for the NP3+ finish.
Black T by Birdsong is a great finish. Not as durable as some though. It's actually two finishes. First the firearm is nickel plated (electroless), then a black Teflon finish applied over that. The really nice thing about that is that if... when the black starts to wear it's still extremely rust resistant because of the nickel plating. It's not shiny at all when it wears so not very noticeable. More a light gray than a true silver.
I have the most personal experience with Black-T and the Glock finishes. I know a lot of people who have guns that are cerakoted, and it's very highly regarded.