FREEZE MAN? He has one pattern: shoot, jump, shoot and subsequently slide, jump, jump! (If I missed a step, just remember: when he runs at you, shoot and slide. It works every time.) Anyway, Burner Man I have yet to beat, Quick Man is pretty difficult, Sigma X3 isn't so hard when you know what to do, and Shadow Devil: I stopped the X series at 5 because it was so terrible, next to 4. (Not that 4 was so bad...)
Kay, so: 1. Bright Man 2. Pharaoh Man 3. Elec Man (Duh!) 4. Solar Man (Especially on hard mode!) 5. Slash Man 6. Hard Man (I guess?) 7. Air Man (Buster only!) 8. Tomahawk Man 9. Astro Man (Buster only!) 10. And... The most difficult Robot Master in the history of the universe: TOAD MAN
Actually, if you wanna see an awesome version of Toad Man, check this out! Some of you may already know this, but for those who don't: Toad Man: Revamped! Stick around for the boss battle!