My argument for Samus being the fastest and most fluid in Super rests on these two things that no other Metroid game has allowed: a run button, and conservation of momentum.
In Super, having the run button available gives Samus a range of speed similar to that of 2D Mario, where you have control over how fast you want to go by holding the run button a certain amount of time. You might want to go as fast as you can, or only halfway there by jumping before you reach maximum speed. There is a good deal of freedom in platforming when you become adept at controlling your speed like this.
The thing that makes that work is the fact that you carry your ground momentum when you are airborne, and you don't slow down until you hit the ground. This momentum is even carried over vertically, so that when you jump while running on an incline, you will jump higher than normal. There is even a trick that consists of running on the ground, jumping to conserve the momentum, and morphing into a ball as soon as you land in order to conserve a higher speed than is normally attainable during morph ball; this is how you can obtain the super missile and the Ice Beam before beating the Spore Spawn or getting the Speed Booster.
There is also another important element that has been in another Metroid game - but not one made by Nintendo. In Another Metroid 2 Remake, you can consistently wall-jump on the same wall, just like in Super. In every other 2D Metroid game since, you are forcefully pushed away from a wall when you climb it, heavily limiting vertical movement.
There are definitely two things that make later Metroid games some fluidity of their own, namely having the Spring Ball earlier and being able to climb ledges (and climbing into morph ball). That said, Super had its own way of dealing with this: you could morph into a ball quickly after jumping, essentially performing the same function, though requiring much more effort.
To me, this kind of depth to the controls is what makes Super more fun with every replay. I never truly master the controls, but always get a little better, and can see the results in my play.