I'm a newer diver than most of the folks on this thread, but I thought I would still share my perspective. I've only tried a few different kinds of regs, and the ones that work best for me have these things in common:
- I can breathe from them in the conditions I dive in, which does not include super cold water.
- They aren't so heavy that they are being pulled from my mouth by their weight.
- The mouthpiece doesn't cause unnecessary jaw fatigue for my mouth.
- The purge button, though rarely used except in dive class demonstrations, is easy to find and push.
- The reg (including octo) isn't so crappy that it is constantly breathing really wet. Not because I can't deal with that, but just because it's annoying.
For me, the ones that have worked best in my limited experience are the Aqualung Mikron (my rec reg) and Apeks Tek3 (my new doubles regs). These regs have small mouthpieces, and the little bridge on the mouthpiece makes it easier to keep in my mouth. Also, they are lightweight (no tugging), can be used with miflex hoses, and have been fairly reliable with few free flow events. The purge buttons on the Tek3 are a little smaller than I would prefer, but hopefully anyone who wants to share air with me can find them.
As a bonus attribute (not a reason to use or not use a reg, but something just for fun) I like that the Mikron comes with a pink vanilla faceplate option. It's cute.
As you can see, I have a slightly different perspective than some of the more experienced divers on this thread, but which information you find most helpful would depend who is in your target audience to sell regs to.