In the small diesel category im going to have to go with the Deutz or lister diesels. [censored] near indestructible. I came across a deutz in a Lincoln welder that had 24,500 hours on it without a major repair. The company actually kept a maintenance log right on the machine.
On the diesel pickup side, i would say the 89-98 5.9L Cummins and the 94.5-2003 7.3L power strokes. I have a pair of power strokes that were bought brand new, an 00, and 02. Both have over 350,000 miles on them.
Medium duty hands down end of story the larger DT series from international DT 466's, DT 530's The P pumped mechanically injected units were indestructible. If the hired hands managed to blow one up even the most catastrophic melt downs were remedied in under a weeks time.
The smaller DT series engines the 360 and the 408 were not as reliable and non wet sleeve engines that were considered throw away Vs their larger brothers that were wet sleeved and easily in framed.
Moving on to the Class 8 diesels. I would say that the pinnacle of OTR cat diesels was 1999-2001 15L 3406E diesels. (Serial number prefix 6nz) Even their big dog OTR motor the 600 HP which was a larger bore version 15.8L also called the 3406E (but with a serial number prefix of 7nz)
Or the Cummins N-14 motors from the same time period. I have to heavily disagree with the ISX engines. Prone to multiple sensor failures and constant EGR problems related issues.
The EPA pretty much ruined the run of reliable diesels in 2003 with their tightened emissions regulations. from there on out it was nothing but turbo failures, EGR issues, broken rocker arms, leaking gaskets and coolant, DPF regen's gone wrong or failed all together, horrible fuel mileage. basically undoing everything that big diesels have worked for all these years.
Also an honorable mention goes to the Brazilian Ford/new Holland diesels. I have a 175Hp 6.6L in a truck that has a light weight of 27,000 lbs. It never has a easy day. The 7.8L were also well regarded and durable, the larger version was also wet sleeved and rebuildable.