I'll start with gun-related companies because that's what the OP asked about.
Gun-related companies:
Choate Machine & Tool -- I might be biased here, because I've known the Choates for a long time, and they're some of the nicest people you could ever hope to meet. That said, I've had a bunch of Choate products over the years and have always been very happy with them.
Beltman -- I only own one Beltman belt and have only used their customer service once, but so far Beltman has batted 1000 in my book. Great belt, which I initially ordered in the wrong size. It was still kind of wearable, so I wore it for a couple of weeks. Beltman still exchanged it without any hassle whatsoever.
DeSantis Gunhide -- I have a
Speed Scabbard for every CC pistol that I carry on my belt. Obviously, it's my favorite. They also make a fantastic bargain in belts, the
Econobelt. It's not pretty, but it's basic black, unobtrusive, and remarkably durable considering that it comes in under $40.
Guns:
Ruger. It's no big secret that I'm fond of Ruger. One of my first guns was a 10/22, and she still sits in my safe today. I'm a long-time fan of the Mark series, too. For me, Ruger often hits the sweet spot in that balance of quality and affordability.
Ammo:
For buying online,
Target Sports USA and
SGAmmo are my go-to companies. I have no complaints with either. For me, the difference is that SGAmmo will let me have the shipment held at the UPS center for signature. Target Sports will not. I'm not real interested in having a porch pirate make off with a case of my ammo, so I tend to go with SGAmmo, unless I can have my ammo shipped somewhere more secure. If I can do that, it just depends on who has the best price.