Well Ray, for me it's been this one...
The 230 flathead six in my '50 plymouth, not that I'm biased or anything, LOL. Driving it daily, lulled along by the sweet sound.
Actually, I Think my favorite six was in the '89 Mercedes 190E that I owned for a few years. Absolutely and perfectly balanced, from idle to redline you could not feel the engine running in the car. Plenty of power, and great mileage to boot. Engineering at it's best.
Pete
