After giving this game a go, I'd put Zero Mission on par with Super Metroid. Shame I never tried it sooner. 2-D metroid games never disappoint, though. The Prime games could never give you the same freedom of movement and agility that the 2-D games do. It's just too much fun. There's nothing else like it.
i got your power bombs right here
What I love about this series is how much it opens up once after a couple of playthroughs. You can run through the game no problem by following the in-game beacons and getting all the power-ups in order, or you can completely break sequence by utilizing all of Samus' fancy movement tricks. Even items that seem necessary like the Speed Booster are optional with some know-how.
It's clearly by design, too, since you unlock special ending slides for doing a minimalist run-through. Which is hard, especially fighting bosses that now take 1/3 of your life in a swing.
You also get a special ending for clearing the game quickly and collecting everything. This requires you to more or less speedrun the game, trying to avoid backtracking whenever possible.
Very fun game that rewards different playstyles and is as difficult as you want it to be.