Got to be some of the BMW straight 6's
Mercedes also had a great 6 cylinder lump thats soldiered on for years.
Neither is the greatest engine in the last 50 years though. I dont know what is tbh. It would be something used in various forms around the world from powering generators and pumps to various forms of transport. If it was for the greatest petrol engine my vote would be the VW flat four but thats nearly 80 yrs old.