From my own experience I'd have to go with the 12v Cummins, and the old air-cooled Listers (I have 5 or so of them).
My vote is for the Benz om617.. Never saw one wear out before the three bodies it's been installed in rusted in half...
60 series detroit, or the volvo/penta in my friends fishing boat.
I'd agree...the older Detroit 60 series (non egr) and the Cummins 5.9 (12V especially) are some damn good engines. I'll add the Cat 3406 B, C, and E engines to the list...very common to see these with a million miles before major failure/overhaul.