Since my primary interest is hunting, I would have to keep my 1963 vintage (with metal 'O' ring yet) Remington 1100, because that is the one gun I own that I have enough different parts for, that I could outfit it properly to hunt just about anything one would hunt or shoot with a shotgun. 26" Skeet barrel, to 30", 3" magnum barrel, with screw in chokes, sling swivels, and recoil pad.
The one gun I would hate most to part with would be my BSS 20 ga. Sporter.