Leon,
I am a licensed firearms dealer in the U.S. and operate a small gunshop/gunsmithing service here in my home town. I have been an avid firearms collector, shooter, hunter, and historian for 19 years. I would like to throw my two cents, or pence, in if I may based on my own experiences and those of my customers.
The first thing I would consider is what gauge you are interested in. While the 20 gauge is perfectly suited to most upland gamebirds and small furry critters, the 12 gauge will give you a wider range of shot choices and killing power for a larger assortment of game. For example, 12 gauge shotguns are used here in the U.S. for everything from quail to Grizzly-it's only a matter of shot size and powder load. I feel that a 12 is a more versatile caliber.
The second thing I would consider is which type of action you prefer. The options are a single-shot break action, double barrel- either OU or SxS, pump-action, and autoloader. Each has is own set of advantages and disadvantages. My personal feelings and the feelings of most of the folks I know is that a pump-action with a 3 " chamber and 24" or 26" barrel is the most ideal. A pump allows you more capacity than a single or double and eliminates the problems/mechanical defects autos are prone to. I seldom have pumps coming into the shop here for repairs, but autoloaders are in all the time for deep cleanings and gas seal replacements.
The next thing to consider is your barrel length and choke combination. A 24" barrel with a modified choke will fit the bill for most game. If you would like to get a little more specific, think about assembling an assortment of various chokes and barrels. A 20" rifled sight barrel loaded with 00 buckshot or rifled slug is a serious deer gun, as-well-as, home invader deterrent. A 28" barrel with a super-full steel choke and # BBB steel shot is a great goose gun. Of course there is a plethora of options in between.
Another thing to think about is the finish. Do you want a flawless, glossy stocked, charcoal blued showpiece or a parkerized, synthetic stocked truck gun. Some manufacturers offer a complete coating of waterproof, camo "plastic" dip. This ensures your gun will be relatively impervious to rain, snow, sleet, mud, etc... although it is still a good idea to dry it thoroughly after each outing. My opinion is that bead-blasted, matte blue metal in a synthetic stock set is the best utilitarian set-up.
Then you have to consider manufacturer. I can personally vouch for both the Remington 870 and the Mossberg 500. Both guns are made very well, have been around for decades, offer endless aftermarket add-ons and upgrades, and seldom need repair. Should they ever need repair, there are dozens of vendors who make/stock replacement parts. I can honestly say that the only thing I have ever repaired on either of these models in 19 years is a single firing pin in an 870. Offerings by Browning, Winchester, and Benneli are also good options, although a little more expensive.
Now for a summary of all of the above mentioned. I feel that for utilitarian use and not for collectibility and/or beauty you can not go wrong with an 12 gauge, 3" chambered pump-action, matte/synthetic, American-made shotgun. I feel that it can ,and has, killed just about everything on the North American continent from dove to moose. The parts are cheap, the ammo is available most anywhere, the reliability is unsurpassed, and they make a sweet sound when you chamber a round. Now for my personal favorite-for both nostalgia, collectibility, reliability, and just plain good looks is the Winchester Model 12 Heavy Duck in 12 gauge! Please realize that these are just personal feelings and the most important thing is what works for you and suits your particular need. I hope this helps and good luck with your decision. God Bless! Jerry