I still love Metroid on the SNES and the Fusion one on the GBA. When I first played Fusion on a DS, I was absolutely floored by how amazing the game really looked. Looking on it through an original GBA and then that blue flip-top didn't do it justice. I only had the GC GBA attachment-thingy for about a week before it got stolen so I never even got to test it out on Fusion. But that was an amazing game and I only wish there were more Metroids made with the same care as Fusion was.
Another reason I liked Fusion was due to the SA-X. It was nice, for one of the FEW times in the series, to know just exactly what had happened to all that bad-ass gear I had earned in previous games. And the SA-X having Super Metroid Samus' gear vs just you in your lowly Metroid-hybrid suit made for some really tense moments when that little fucker would pop up out of nowhere and slowly stalk around the room, searching for you. Rarely does a Metroid make you shit your pants like that. Plus you got a really nice boss-fight vs the SA-X with all your old swag still on it, so that was an extra plus.
If I'm not mistaken, the SA-X still had the Spaser Ice beam, the Super Missile, the Screw Attack, and the power bombs?