Current or all time?
Current (last 10-20 years) production guns it's hard not to go Benelli SBE line in whatever generation you prefer.
All time, I'd hazard a guess more ducks have fell to a model 12, 870 or A5 than any other guns ever will.
I've beat their drum before but a dark horse contender is the franchi affinity. Benelli inertia actions, but also made in Italy instead of Turkey like the stoegers. And I don't think there is a simpler gun to disassemble and clean, not that it needs it very often.
I am currently in the market but I can't make up my mind. I am sorely tempted to get a new A5 in 16 gauge and make it my do everything gun for a while. Doves, quail, Kansas upland for quail and pheasant and ducks…
I also really like the look of the new redesigned montefeltros. One of my LGS had a SBE 3 in 28 gauge for a really decent price but somebody finally removed the temptation on that one
Decisions, decisions…