I love the Metroid series too. I once tried Zero Mission because I wasn't interested in Metroid, but wanted to try one to see how it was, and I absolutely loved it. Every single moment. I love the music of the Metroid games, I love the amazing atmosphere, I love the Prime's stories etc.
My favorite Metroid game would have to be Metroid Prime by far. It's also my favorite game ever created, period. It's just amazing. And believe me, there are many games and I talk highly about a lot of ones, but deep in my heart, I don't mean it as much as when I talk about Metroid Prime. Every good word I say about it, I mean the full 100% and then some. Metroid Prime is a masterpiece, a work of art and a legendary first attempt from Retro Studios. How many developers can say their first game is THAT good?