BMC A series? BMW? GM v8?
What are your thoughts...

l think it is the one they built to fly to the moon.

Really u need to specify what ur asking? Best could mean a lot of things. Like some can be built to huge HP for little cost like the Chevy 350. Some r just real power houses to start with. Some r unique that they can be installed in a lot of other cars easily. Some r the first of a huge radical change in engine design & r a legend for their historical significances.

As to the BMW in ur question the best over all most reliable long long lived little attention needed is some of the M30 motors. the 3.3 & 3.5 8 to 1 compression motors. BMW made that motor for years. But then ruined it by slightly changing it to a 10 to 1 motor. The changes only gave it 12HP more & a little torque. But at a cost of cracked head, valves hitting the pistons on high revs, the demand for 92 octane gas & some other stuff. l have owned & worked on a lot of cars over the years & stopped when l found the OLD BMWs. Over all they r still more dependable than newer BMWs. l would jump in an old BMW before a new one to drive across the US. Because l feel better that l wo not be stuck on the side of the road. SOME cars time & miles have no effect on.

Probably the 3 liter, 5 cylinder Mercedes Benz OM617 diesel. They commonly last 500,000 miles

The BMW M10 is definitely one of the best as well.

None of the above. Boxer Engine is are probably the longest lasting due to their positioning under the hood. The oil basically surroudns the engine, even when turned off. Only two car makes that make these are:
Subaru and Porsche.

That is why you see people with Subaru is with 300,000 miles on them.