This question is highly subjective. To me, "best" means that if I had a car from that decade, I would wish it had this engine in it:

70s: the 6.9 M100 is awesome but expensive, low revving, and paired with a loaf of a trans. In the end I think I'd take an injected M110.

80s: M117

90s: M113. Even though I think the M120 is really cool, I can't call it better than the M113.

00s: M113, or the AMG M156

10s: M159

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'82 300TD
'77 6.9
'75 280S
'74 280
'87 Porsche 944 turbo