I absolutely LOVE the Metroid series! I didn't start playing the games until Metroid Fusion came into it, but that was such an awesome game. I enjoyed the first two Prime games - especially the second, both the Dark Samus battles and the entire landscape of Sanctuary Fortress were incredible - but I felt it ended a bit poorly with Corruption. I really didn't see the need for all the unexplained and undeveloped characters (much as I love Gandrayda's design) in it when Samus alone would have been sufficient.
That said, I detest Other M. Samus' portrayal in that was so painfully sexist that I traded the game in immediately after I'd finished it - it's the first time I've ever been happy that a game has been so short and easy that I've finished it in under eight hours. Other M completely tore down the picture of Samus as a strong and independant female protagonist that had been built up over the years. I wouldn't even consider it a part of the series.
...the cameos were nice, though. Seeing the Nightmare was both enjoyable and painful, because that boss gave me so much hassle in Fusion...
I wonder if they'll ever release Metroid Dread in some format...it'd be nice to see it for 3DS. I'm pretty sure the story was fully written, so perhaps they'd use that for a 3DS or Wii-U game? Although if they're going to make another Metroid game, I'd prefer to see something either like the 2D games or the Prime games...if the next game is anything at all like Other M, I am never touching the series again.