I just finished another playthrough of Super Metroid last night. Every time I play it, I find myself marvelling at how tightly designed it is. The formula is really fairly simple, but the game has so much depth. You can experiment with different combinations of equipment, turning it on and off as you please. There are those secret little things like the Crystal Flash and the charge bombs. I love how the etecoons sing the "item acquisition" theme when you first meet them.
I love every Metroid game except Other M. I didn't care for Metroid Prime the first few times I played it. I think that was because I was at a friend's place with him looking over my shoulder, and I didn't really feel free to explore and play at my own pace. Once I picked up the game and played it alone, I fell in love. I really like Prime, and I like Echoes almost as much. I didn't care for Corruption too much, but I've only played through it once and need to give it another shot. I actually like Hunters quite a bit, but it's not in the same league as Prime and Echoes.
I'll never understand why Nintendo doesn't release more sidescrolling Metroid games. They use Mario and Zelda as their go-to series, but they forget about Metroid. It really seems like Metroid/Zero Mission and Super Metroid are the only genuine sidescrolling games that stay true to the formula. Fusion and Return of Samus are much more linear, and all the others are 3D. I'm dying for some more authentic Metroid...