My one-gun choice would be a light, 7 lb or somewhat over, Perazzi MX8 12 ga game gun with 28 3/8 barrels and choke tubes. For several years, I happily shot only a Perazzi similar to this, primarily on low-key, non-competitive clays, but also for all my then limited upland hunting.

I would not envision water fowl, deer, or self-defense use if I were restricted to a single shotgun.