Gotta be Super Metroid > Metroid > Metroid II > Zero Mission > Fusion.
The original is good despite its rough edges. Its short length and relatively higher difficulty makes it the most replayable, with its only real chaff being the opening Brinstar area due to being a bit slow on replays. I think it telegraphs its secrets more strongly than people tend to give it credit for, and is a very solvable game even if playing blind. Its stronger focus on resource management is really fun too. I don't blame anyone for being put off by the pants-on-head dumb 30 health on respawn thing, but I think this is one of the best games in the series.
Metroid 2 is totally different but still awesome. While Metroid 1 is flawed, I think Metroid 2 is great and easily recommendable without any real qualifications. Its map layouts are really intelligent and feel very thoughtfully built around the player not having a map, keeping that from being an issue as you might think it would be, and though the Metroid hunt isn't quite as cool as the search for the bosses in the first game due to its linearity, the game still evokes feelings of a great, dread-filled adventure.
Super Metroid is of course the best. The structure is basically completely different from the previous, more open games, but is still executed ingeniously. The relative lack of difficulty is a bit of a shame but the game does make up for it in other areas.
Metroid Fusion has a lot of amazing aesthetic elements, just as Super did, but falters in other areas. The first three Metroid games build and evolve on each other in really interesting ways, but Fusion just feels like a warmed-over Super Metroid rehash without any real improvements or new, great ideas beyond the (initially exciting, to be fair) novelty of the SA-X sequences. Super being an entire game full of the "get new item to expand the areas you can go to, explore the new areas until you find the next new item, repeat" concept that actually isn't used that much in the first two Metroid games is fine because that game is able to execute on that structure with incredible deft and cleverness, but Fusion repeating the exact same beats - but now as an even more linear adventure, and without any real meaningful improvements to combat that you would expect would come by streamlining - makes it feel rote in comparison.
I have the same criticisms towards Zero Mission, though I appreciate that Zero Mission at least isn't as completely gated and linear as Fusion (though it's not really much better). The first Metroid is a very different game than Super Metroid, and despite its poorly thought-out elements, it appeals to the player in very different ways. Zero Mission just feels like a simplified, worse Super Metroid that's been pressed into a shape vaguely resembling the original Metroid. I feel the same way towards Samus Returns - though it's enjoyable, I don't think it captures the appeal of Metroid 2, and it isn't as well-executed as Super Metroid, either.
I'm hopeful for Metroid Dread but I don't think the Metroid franchise / concept is one that lends itself well to repeated iteration without major shakeups. I enjoy even the weaker entries in the series but I don't have a ton of passion for them.