I didn't even know of Metroid II for ages - I had assumed it was a Japan-only release or something, I was surprised when I found out it was on the Gameboy as I don't know of any other series with direct sequels spanning both handhelds and consoles.

I'm like that with Legend of Zelda, though - the first one I ever played was Link's Awakening. I loved it, and was surprised to learn that it was regarded as a weird and uncomfortable entry to the series.