The slant-six, 225ci. I had one in a '63 Valiant (modded to about 200hp) and one in a '66 Coronet.
Many "hot rod" parts were available since Plymouth had planned to use that engine in a sport car, which, obviously, never came to fruition. All that remains of the project is the prototype XNR (for Exner, the designer).
Remember the Honest Charlie Speed Shop? Their catalog contained all the performance parts for the 225 - Edelbrock, Weiand, Weber, etc. Intake manifolds were available for 2-2bbl, 3-2bbl, single 4bbl, dual 4bbl, or triple Weber sidedraft. A 6-3-2 exhaust was available, as were larger valves, better rockers and springs, hot cams...
I wanted the Weber setup for my Valiant but cost was an issue for a high school senior, so went with three Carter 2-bbl, slightly hotter cam, larger valves, and the special exhaust. Spent a small fortune having the head work done. Larger radiator, homemade CD ignition, electric fuel and oil pumps, floor shift (can't be cool with a three-on-the-tree) ... and parents scratching their heads, "what's wrong with that boy?"
It was still a nerdy four door Valiant, but oh it ran nice, and sounded better.