I was planning to pretty much go with the Prime series after Super Metroid and Zero, however now that the Prime games I would have voted for already won, I went with Fushion. I believe it is an excellent game that really added to the Metroid experience. Some didn't like the less isolated feel, but I really enjoyed the nature progression of the story and how it felt like Metroid 4, and the changes incorporated to make it more accessible for portable gaming.
The only thing I didn't like was the Spider boss.