Ooo, how exciting! If you haven't already tried it, I highly recommend Super Metroid. IMO it's the best in the series, and arguably the best game on the SNES. It's comparatively obtuse though (though not nearly as much as the original Metroid), so if you're new to Metroidvanias in general then maybe try Metroid Fusion first. It's a bit more hand-holdy.
I did play a bit of Fusion a while back, but never got very far, so I don't consider myself to really have 'started' with the series lol. If I ever find my 3DS again I may restart it. Or, uh, emulate.
I was planning to play Super Metroid after trying Metroid II (and probably after beating it), just for the very thin threads of plot. Even though I already kind of know the plot of SM.
I recommend playing all three. Not necessarily in a row mind you but at some point. Metroid II is a little rough and dated now but I appreciate it all the same. It's a little obtuse still but it isn't a very big game.
Samus Returns I actually really liked. AM2R I have some reservations about but it's a really solid fangame regardless. But they both share the same problem the Game Boy original had: Too many goddamn Metroids. The grind gets old in all three of these games.
I feel like I've got a decent enough tolerance for old, obtuse games, so I may try the GB one (especially since I started with the NES Metroid). At the very least it'll help me appreciate the others when I eventually get round to playing them. (Also I'd have to buy Samus Returns because I don't have a good enough computer to emulate 3DS stuff at playable speeds.)
AM2R now includes a Randomizer (with logic to prevent soft-locks) and has since been updated to no longer require beating the game to access it.
Maybe I'll try it when I get a better feel for the series!
But yeah, thanks for all the tips, guys!
I also looked it up and didn't realise that apparently Zero Mission has a whole extra section at the end of the game? I forgot how fun it is to jump into a new series.
As an update, I beat Kraid earlier today. I did use save-states so that I didn't have to go through the whole lair every time I died, but I did try to do the entire thing in one 'take'. Ended up winning with so little HP that attempting to leave killed me. But I won, so whatever! Guess I gotta find Ridley and the rest of the items next.