Mine is probably going to have to be a tie. The Buick 455 I had, which had enough torque to pull a house off its foundation and loaded 18 wheelers out of snowbanks. May only have gotten 13mpg, but with a Edelbrock 750 carb, TA performance headers and dual glasspacks, even in the land yacht like the Electra 225 it would just flat out take people by surprise. Its just something about an engine with that much torque that when you sit in the car, put your foot to the floor, and feel the Earth being rotated off its axis that makes you just feel like superman.
Then of course, the other very obvious choice (for me anyway) the 87 LC2. Aka the GN motor. Its like a sniper. You never know its there. Doesn't give away its true potential, but once you pull the trigger its absolutely deadly.