No votes for the Mercedes 240D, 300D? Those have to be some of the most bullitproof engines I have ever came across. Not great on performance but man those things last forever. I cant tell you how many of those things I maintain in my shop with well over 300k on them and still going.
I just hit this thread and haven't got to the end yet, but that would be one of mine.
so before I finish the thread, I'll put in mine:
"All time is a bit difficult, because we might have to add some historic engines, like the Model T, the Ford flathead V-8, the Bugatti Royale, the Offenhauser, etc., but for ordinary, relatively contemporary engines.....
Mercedes 200-220-240D family (normally aspirated OHC 4). My 240D was still ticking like a clock when I gave it away at 325K miles and 28 years (rust et the rest up!). Brilliant design and quality make a luxury car and the world's standard taxicab the same vehicle.
Chevy 350 - kind of like the quintessential "detroit iron."
Jeep 4.0 of course - the culmination of the great American OHV six.
VW flat four. OK, not really the toughest thing around, but a work of genius in its own way.
Mopar slant 6. Automotive and industrial standard. A hundred years from now, someone will be running some kind of pump, truck, van, compressor, siren or snow cat with these.
Runners-up: Almost every other American OHV straight-6. I lament the passing of the great American straight-6. With the possible exception of the egregious OHC Kaiser, has there ever been a bad one?