As Tony, my favorite is resin finishes. Currently I offer GunKote.
It is the best *metal* finish I have ever used.
Thin (can be applied to internals), self lubricating, durable, impervious to solvents and looks great. Oh, it also aids dissipation of heat. Properly applied it will never chip/flake. It wears like park on steriods.
Duracoat is an epoxy based coating, thicker, but more flexible. Best for synthetics, wood, and parts that can't be heat cured as the GunKote resin requires (like scopes)
(IMO) Powdercoat is not an appropriate firearm finish. It is a strong paint, but like any other *paint* it bonds stronger to *itself* than the substrate. Thus it will always eventually chip/flake. AND it is not immune to all solvents. It is also too thick to apply but to only the outside surfaces.