I'd have to disagree with all of you and say it was the original 1960's inline four designed by Alex Von Falkenhausen. They built that engine and variants of it for twenty years and it powered the cars that turned BMW from an unknown into the successful giant it is today.
It was a very simple and strong design - I've rebuilt a couple and they're remarkably simple to assemble. How strong? Well it's the same block that became the turbo F1 engine.
The modern BMW engines are superb pieces of engineering, but not one of has stayed in production for 20 years. Yes, that's because of changing standards and so on, but still, the old four-banger made BMW what it is today. It wins!