Mullenkamp

An interesting blend of classic Castlevania and the later Metroid-styled ones, with lots of hidden goodies in every stage and reasons to go back and search for them. Hard, but satisfying. The aesthetics and the OST are both fantastic, too.

One of the best beat-em-ups ever made, with tons of secrets and neat stuff. Has branching paths and optional areas, too. An interesting note is that it was actually written up as a traditional tabletop campaign as part of the development process.