My all around choice would be a Beretta 390/391, Browning Gold, or other quality gas operated semi-automatic with 28" barrel as it would have the heft for a full course of sporting clays or heavy pheasant loads while being light enough for hiking all day after grouse. If grouse were the main course and the others only occasional then I might settle for something light like a Franchi 48 or Benelli Montefeltro and suffer through pheasant loads and sporting clays.
I got by with the former for years but have come to appreciate having guns better suited to the task at hand. One can get by pretty well with a jack of all trades but the rest of the saying is true too- it is a master of none.
If I could only keep one shotgun it would probably be my Beretta 391.
That or the 391 would probably be my choice too if limited to a single shotgun. And that is a hard thing to say from someone who has to pay full price for a Beretta. I really like my Browning Gold too but the Berettas have given me a lot of hard use without a significant hiccup. With a 28" barrel the stock 390 will even cycle Winchester Featherlites which are a little less than 7/8 oz of shot at under 1000 fps. With the heavy spring from Cole's the lightweight load will cycle through a 22" slug barrel. The 391 will cycle the Featherlites with a 26" or longer barrel as it comes from the factory.
The only thing I truly dislike regarding the Montefeltro (and original A-5) is the need to make sure the magazine nut is tight to the fore stock otherwise you run the risk of splitting the stock. I only have A-5s to worry about but I have found the stock on them and the Montefeltro can catch on the side of the receiver no fool you into believing the nut is tight. Recoil then shifts the fore end into the proper position but with some slop. T some point the stock will crack and that can be a tough fix.
