I loved Other M's Nightmare fight. It was a really unique boss battle that really stands out in the midst of the other, more generic bosses.

The one thing I really liked about Other M was Samus's animation in the gameplay. She was really smoothly animated and her movements screamed agility, especially with the wall jumps and dodges.

My reason for loving Prime 2 is its atmosphere. Prime 2 was very dark and gritty, and I love gritty games. And Sanctuary Fortress is just jaw dropping. I also feel that Prime 2 has the best boss battles out of any Metroid game. They had massive scale and were more puzzle-oriented than in most Metroid games. But yeah, they're all great games, with Other M being at the bottom despite being fun.