Currently, hard chrome is STILL the best all-around gun finish.
It's combination of hardness, corrosion resistance, wear resistance, thinness of the coating, and options of appearance are the best.
It's HARD.
Actually harder than a Swiss jewelers file.
The "slick" hard chrome prevents fouling from sticking to the surface, so cleaning is much easier.
The slick finish actually improves the trigger pull and operation of the mechanism, and extends the life of the parts due to the hard, wear resistant coating.
Corrosion resistance is very good, and it's proof against all solvents, sweat, most acids, and moisture.
Wear resistance is great.
It's so tough, it's one of the few coatings that's considered to be a genuine "life time" finish.
Since it's molecularly bonded directly to the steel, it can't crack, chip, or peel.
Scrapes that will scratch or break through most other coatings will only leave a shiny scratch on hard chrome.
Hard chrome is one of the thinnest coatings, so you don't have to worry about tolerances being changed by a thick coating.
Appearance can range from a dull pearl-gray, to a near nickel plate-look.
Each plater offers differences in cost and appearance options.
Here's the top platers doing gun plating:
http://www.originalmetaloy.com/
http://www.fordsguns.com/index1.htm
http://www.apwcogan.com/Greetings.htm
http://www.trippresearch.com/
http://www.techplate.com/firearms_pl.htm
http://www.mahovskysmetalife.com/