My favorite engine of all time is BY FAR the B series Hino diesel that Toyota used in their B series Landcruisers. They are bulletproof, low maintenence, very reliable, clean burning and economical to run. They have no timing belt, mechanical lifters and an oil lubed inline pump which is very tolerant to poor quality fuels and neglect. I have had numerous B series landcruisers from a 1978 2B to a 1989 13B-T direct injection turbo. They are all great and I take good care of them and they have done wonders for me. A close second is the Mercedes OE616 and 617 diesels. considered by many to be the worlds best built motor. Maybe they do deserve that title however, they are higher maintenece than the B series, and they have a timing chain instead of gears. Nevertheless, they were excellent mills!!! Then there is the 2.2 izuzu that was mentioned which was also outstanding. And the 2.2 perkins that Ford used in their rangers prior to 1985. I had one of them with 870,000km on it. Motors like the little L series 2.2 and 2.4 toyota pickup diesels and 1.6vw diesels were great on fuel and reliable as a horse but they did have a timing belts and mixed metal engines. Good motors but, not the best in my books.
Then when it comes to North american stuff, I like the 6.2 in light applications and IH idi's and early 12v cummins in heavy applications. But any engine that was known for possible head gasket issues, dropped valves due to bad guides, dropped "death pin dowels" or injection pumps that only last 150,000miles is not even worth comparing to some of the better engines out there. Especially when fuel economy is more important now then it has ever been. And I don't even consider post 1994 stuff when im thinking good engines...just too much electronics and this is the era when vehicles were starting to be really designed not to last, and its gone down hill from there. Look at the diesels of today...what a joke. You got to pull the cab off of the Fords to do any engine work, which they need a lot of I might add. Dodge and GM aint far behind. and with todays technology and for the price we are paying for new vehicles; we can be building a truck engine that gets 40+mpg no problem and small engines to get probably close to 100mpg. People who keep falling for this new stuff got to give their heads a shake.
Then the commercial stuff is not fresh in my mind but I have always been a Mack fan, E6350 is what I have now. I liked the early cummins 350s etc. Detroits are just cool in my mind, don't know if I were to buy one again to run more then occasionally due to them beign just too high strung for me and they like to drink the oil, which aint cheap anymore either. I also run an early 425 CAT at work, been a great engine to. We have a early 2000's kenworth with a C13 CAT in it, spends more time in the shop then working. Way too much electronics and garbage on them. They are good on fuel however.
And as for gas engines, the old dodge slant 6 was just too good to be true, ******* fuel but reliable. I have heard of people getting many miles out of vortec and triton engines also, but have heard just as many if not more horror stories also.