The one that allows the original NES Metroid to save your progress instead of depending on a bogus password system.
In a similar fashion above, the one that not only allows you to save in Castlevania II, but makes the game much less cryptical and far more manageable, with actually accurate clues as to what to do to advance.
Almost every undub there is. Most old-school dubs for foreign games suck, especially JRPG's, so I always seek the undubbed versions of those games.
As for individual games that expand on the original iterations, I suggest Super Metroid Redesign, The Legend of Zelda: Parallel Worlds, Super Mario 3Mix and Final Fantasy ++.