Thinking of starting my first Metroid game, what should it be?

[Zero Kirby]

Super Metroid is a good place to start, though it's worth noting it's the last 2D game to have "floaty Samus" since the future 2D games all feature much tighter gravity. I'm not a huge fan of floaty Samus but Super Metroid as a whole is so good that it balances out.

Me personally, I started in the series with both Metroid Fusion and Metroid Prime, which is also an excellent jumping-on point and depending on who you ask (i.e., me) is even better than Super Metroid.