I feel like it's bold of a sequel to at least sort of admit it can never be "Super Metroid", so Fusion here doesn't even try to be. It refines the movement to give Samus a little more finesse and the ability to be a little more mobile and makes things like wall jumps and space jump a little more consistent, but to make up for it, it cranks up the difficulty a lot. Samus in Super is a freight train of pain, Samus in Fusion is comparably VERY fragile, which really works for the horror-lite angle of this game. Even the health and ammo drops are trying to kill you in this game.

Speaking of horror, SA-X is a very interesting antagonist, getting to essentially be Samus in her prime but as a much colder, animal-like killer that you absolutely do not stand a chance against for most of the game. In execution, she turns up at set moments in the story and only one chase sequence is forced and is even all that difficult, so SA-X just isn't that scary to me. Though maybe I'd sing a different tune there had I played this when I was a kid and this game was new.

It's comparably very linear to other Metroids, very little being able to be sequence-broken, but it serves multiple story purposes, both in how there's a very set sequence of events to go through in the story and how it's reflective of the Federation keeping Samus under their thumb. It's claustrophobia but from a completely different angle, which then makes it gratifying when you get Screw Attack and can finally freely explore the station on your whim. To which they include a lot of tricky puzzles to practice. This does inevitably mean it has a "lap around the world" issue unlike Super, but I'm not sure how else they could've done it.

Mine can be considered an hot take, but.... I think Fusion is the best Metroid.

Many may consider it a step-down compared to its predecessors or the title that came after it, and I can see why: specifically because it drops the maze like structure of the map in the past games, for a more linear exploration, that can make the Research Station a bit too "Videogamy" compared to Planet Zebes and the other iconic locations in the series.

But this doesn't stop the gameplay for feeling immaculate, especially for a GBA title: Samus' moveset is fluid and leads to a gameplay loop that lets you move around the map in a thrilling way, and lets you enjoy the various quality in the map sections, enemies and bosses.

The biggest aspect of Fusion for me is its writing: Samus reactions to ADAM and the corrupted decisions of the galactic federation leads to the best characterization for the bounty hunter in the entire series.
The terrifying sequences and enemies you encounter literally turns this title into a horror game, that will keep you on your toes as you fight nightmares reminiscent to Jon's Carpenter's The Thing.

(side note but it also confirms that Samus canonically saved the animals at the end of Super Metroid, so that is a giant plus for me :D )

Absolute banger game. Highly recommended!