For my part, I'm VERY fond of 16 gauge. I lean heavily toward 12 ga when hunting tough species like pheasant and sharptail. 12 ga is, IMO, the only way to go for most competition sports. Conversely, I appreciate the lively handling and lighter weights of sub gauge guns when facing a long carry in hot conditions while hunting small, nimble species like ruffed grouse and snipe.
Back when the shotgun bug bit me hard, I was somewhat overwhelmed by the variety of makers, types of actions, and gauges. It was daunting to consider how much it would cost just to acquire one of each, let alone a decent collection. A wise and dear friend bailed me out with a good piece of advice. "Don't even try to do it all", was his advice,"choose areas of interest and focus on those." By this, he meant to choose British guns, or Spanish guns, or American guns; choose break action doubles, or pumps or semis; choose 12 gauge, 20, or .410. The ability of this approach to focus a person's efforts, and to offer ultimate flexibility for your own personality to come out is powerful.
Using myself as an example, I couldn't decide whether I lusted after British, American, Spanish, or other makers. I wasn't sure of my preference of action designs, either. But, I was pretty sure I didn't need every gauge that was ever made. So, I started with only one rule - when it comes to shotguns, I would confine my purchases (shooters AND collectors) to 3 gauges - 12, 16, & 20. I've adhered to this policy ever since. Have I ever been tempted to buy a 28 ga or a .410? Most definitely, but I didn't.
Since then, my tastes have narrowed even more. I've discovered that I have no use for semi-autos (my personal taste - no offense to those who appreciate them). My opinion is that the nationality of the maker is irrelevant; often the maker doesn't matter nearly as much as the quality of the individual piece.
The result of these choices is that all of my shotguns are break action or pumps; all either 12, 16, or 20 ga. Even with only the 3 gauges in my safe, I have found that 16 ga has not been orphaned by 12 and 20. My choice of gauge for any given hunt usually revolves around the shot size(s) I intend to use and the load. When I'm going to be using 6 or 7 1/2 shot in one ounce loads, 16 ga gets the call. Choosing the gun based on the shot size and load has never left me feeling under-gunned. It has never left me feeling like I'm carrying unnecessary weight in the field, either.
I've been very happy with the choices I made so many years ago. I've never once felt deprived of flexibility because I don't have a .410 or a 10 ga. The 16 ga guns don't sit in the safe because they seem like a half-step between a 12 and a 20, either. On the contrary, they fill a sweet spot where 12 ga would mean using a light load, and 20 too heavy a load. The best all around performance from a one-ounce load, for example, is achieved with a gun in the 6 lb, 8 oz range. By performance, I'm referring primarily to recoil and shot patterns.
16 gauge guns are a poor choice for competition sports only because competition loads are almost impossible to find. If a competitor were to load their own, however, 16 ga has a patterning advantage in the load ranges specified by the governing bodies. For hunting purposes, I've never had trouble finding suitable 16 gauge shells. I even have Gamebore 2 1/2" loads in 16 ga. Supplies can be limited in some areas where retailers don't see enough demand, but even those retailers will special order by the flat.
I realise my opinion contradicts that of some who have had different life experiences. And, that my feeble old mind may have strayed into the realm of story telling. Please feel free to ignore my comments if they cause you to feel offended in any way.