depends if you want auto, pump, or o/u...There are many choices in both pump and auto. The most reliable and close to the cheapest would be a Remington 870 deer/field combo. Semi Auto is going to be more expensive but less recoil and a little more maintenance required (cleaning). I too am lokoing for a new shotgun for waterfowl and deer hunting. I think I am going to go with the 870 and get a good recoil pad to cut down on the recoil.
I use a 20Ga. 870 on upland birds and Rabbits and an 870 SP Mag for Deer, Geese etc.. If had to go with only one, think it would the 20 Ga, mainly because of weight.
There are two models of 870, the Express and Wingmaster. The Express tends to rust if you don't good care of it. Personally, I would stick to the more expensive Wingmaster. I do own a Express and without knowingit rusted rather quickly while being stored away.
Take a look at the Beretta 390's. Wal Mart has these in a black matte/composite with a 28" barrel for about $525. I've had one fora whileand use it for grouse/woodcock, pheasant, ducks, turkeys, deer, skeet, trap, an sporting clays. Very reliable, soft shooting, and easy to clean. Have thousands of rounds through mine, and has never, ever failed.