Based on experience only, the engines with which I am familiar are the M100 (in a 6.9), and the the M112 and M113.
After owing a 6.9 for 15 years, only the front main seal , the harmonic balancer and the timing chain were replaced. Since then no further work has required to be done by the next owner who has the car on the road, and driven regularly for the last 4 years, occasionally on interstate trips.
As for the V6 M112 (in a W202), the valve cover gaskets and rear main seal have been replaced in the period of my ownership of the last 4 years.
Car have travelled 225,000 klm. I have replaced all 12 plugs.
I have two M113's an AMG 5.4 liter and a 5 litre in a CLK. coupe.
No work at all so far on the 5 liter, but it has travelled only 101,000 kilometers. However oil seals deteriorate as a function of age, not distance covered, and I will consider the idea of a preventative maintenance decision to replace all oil seals sometime in the not too distant future. I took the decision to replace all 16 plugs not long after I acquired the car
The 5.4 AMG engine in the W210 is 23 years old and has also covered only 101,000 kilometers. However I have already taken the decision to replace front main seal, the harmonic balancer, the rear main seal and valve cover gaskets and all 16 plugs for preventative maintenance reasons.
In terms of reliability, the M100 engine tops my list.
But I will need to own the other cars for at least 10 years more to consider them more highly, but based on what I have read on other MB forums, the M112 and M113 engines, if maintained regularly have been considered by others to be "bullet proof".
Not quite so much so for the later M272 V6 and M273 V8, (which are a quad cam development of the M112 and M113), that had timing chain sprocket gear problems, which can distribute very fine metal particles throughout the engines from new. as the teeth on the sprocket wear out prematurely. They also have some problems with the plastic flap valves in the induction manifolds, parts of which risk getting into the combustion chambers.