The " best" all round gun id the one that fits and you shoot best.
Thankfully I don' t have to settle on any one particular shotgun. IMHO the " best " is a O/U if one absolutely had only the choice of one. It would be in 12 ga for the versatility and also availability of loads for it. The 16 is a nice choice but ever try to find ammo in a particular load that you want? Not an easy chore where 12 ga can pretty much be had in whatever you want whereever you go.