Super Metroid. It was really got me into the series! Never looked back since. To be honest I don't think Other M is nearly as horrendous as everyone makes it out to be, but then I thought the game play was quite solid and could care less about the story. Still, not the greatest..
Super Metroid is my all-time favorite game, so, yeah.
Fusion could have been a classic too if it hadn't been for the massive amount of text in the game.
The first Metroid is pretty sucky, though. Without a save system and a map, the game is pretty unforgiving. Zero Mission was a much needed remake. Although, Super Zero Mission is even better!
Anyway, I voted for Fusion, though I own and have completed most if not all Metroid games that have been released. Only exception being Other M. Just haven't played it yet.