For all the guff Metroivania's get, the love for Metroid's is ridiculous. Every damn game is the same and they all of them even bother to recycle the same damn weapons every time.
I dunno. Super Metroid is great. It feels like the original game DONE RIGHT. It makes you remember the first game but also makes it so much better.
Metroid 2 for the GB is a rather different game. Some people like it best, but the lack of color made it difficult for me to play. I cannot fairly comment.
Metroid Prime is very different. I've shared my conflicted ideas on the game in the post above. I have not played the sequel, mainly because I had no GC and I have written it off as inferior due to the comments of others.
Fusion is a very nice game, and while it includes a lot of the things from previous games, it tosses in a lot of new ideas and weapons, too. It has a more level-y structure and is still pretty challenging. Probably my favorite in the series, although nostalgia makes Super look very nice.
Zero Mission is a good remake, but too easy. At first it looks like all roses, but then you notice it has a Metroidvania difficulty level.
Overall, a good series. The 3d games are the only real missteps, and I don't think they are so awful that they can't be corrected.