One of the rarest muscle cars, the 1967 Ford Galaxie 500 with R-code 427 V8 engine only had 24 examples made. If you find one, though, it's pretty worth it.
In the 1940s, Jaguar created its XK6 inline-6 engine and continued using it for nearly 50 years afterward. It wasn't just powerful, it was nice-looking to boot.