Funny thread. Me I would say diesels. om617, om603, om606, om648.
What ever was earlier was probably awesome to. I was always eh on them before pretty recently. yeah the 617 was a run for ever work horse But in the last couple of years I have been playing with them and I am making around 700ft/lb wtq with a stock om606 and have a 648 swap in my own car. I threw the 603 in there because after it has a good head or never gets warm they really are quiet and smooth.
Gas. The 6.3 in the 60s.. Just awesome looking engine. 70s the mechanical injected inline engines are all great. 80s 16v. 90s m104. 2000s m275.