1. Other M
2. Prime & Corruption
3. Fusion
4. Echoes
5. Hunters
(SPOILER ALERT[possibly a little bit])
I think Other M is the best in the series, despite some gameplay flaws. Samus's character is finally developed and she is not only interesting but realistic. Finally we have a game where samus doesn't just stumble and lose all her powers. There is a good reason for her not to use her more advanced weaponry, a superior officer who's command she is acting under has restricted her use of these until the extreme situations call for their use, to me that presents a reason for the game to restrict your powers that's realistic and inline with the story. We get to see the Samus Nintendo had invisioned from the start, one who is human and full of fear and anxiety about the life she's been thrust into. People saying she wouldn't hesitate upon seeing Ridley because she's killed him before have no understanding of what real combat is like and what stress and anxiety can do to ones thinking and behavior.
Think about it, she's seen this monster kill her family before her very eyes. Then as an adult she kills him four (? not sure the exact number of times before other M that you fight Ridley) and he just keeps coming back to haunt her, of course she's gonna freeze up. Also all of the thought put into the title and setting were great, Metroid: other M; abbreviated MoM, Samus's character spends a lot of time lamenting the baby metroid that gave it's life for her, a lot of emphasis is put on mother/daughter relationships (samus and baby metroid Madeline Bergman and Melissa Bergman) the "baby's cry" distress call from the Bottleship that's shaped like a babys bottle. All of it well thought out and not just some generic sci-fi setting and some generic sci-fi one dimensional cartoon character hero.
