Seems like to be "best" it has to have been in use for a very long time, reliable, and has been upgraded in its basic form for many years . The small block chevy and all its derivatives have met that definition for longer than just about any engine IMO. There have been a lot of great engines named on this thread but none have had the longevity of the small block chevy.
yep, seen a show once talkin' bout how many (100 million?) or some such were made... got the original 265 in my 55 Bel Air sport coupe, still runs good... like to build an L79 327 (350 HP) for it... cant bring myself to rip that old mouse outta' there......