Best motors ever built? On what criteria?
The 60* V6 motors from both OMC and Mercury were real good in terms of power and reliability. Of course they were simple carb 2 cycle motors. They were made from 1977 until at least 2010, with the later models being e-TEC and Optimax versions.
The Big Twin motors from OMC were real good. They were made forever, or at least it seemed like forever.
Merc il6 motors were made a long time, and were part of the 2, 4 and 6 cylinder inline family that shared parts. The Il6 motors were not as good as the V6 motors from Merc, but were significantly lighter.