An inline 6 such as those that Ford used to make and General Motors and BMW still makes. An inline 6 is inherently much better balanced and smooth than any 4, V6, V8, or V10.
A little trivia. The firing order on most inline 6 engines is 153624. The way we were taught to remember that many years ago was with the little ditty "fifteen is too young, thirtysix is too old, but twentyfour is just right".
