For all around durability you can't beat a 390 Ford or a 350 Chevy. As long as we're talking 4 wheels.
But...
The best engines I've ever owned have been in Goldwings. These things just don't quit. You can ride it like there's no tomorrow, put it away wet and do it again tomorrow. They'll easily do 200,000 miles. You'll have to replace the seat, shocks, wheel bearings, etc. but you won't wear that engine out. My 1800 EFI 6-cylinder is a dream come true.
