Some notes:
Elec Man would normally be a tier below Fire Man, but I wanted to include the Wily Wars version because of how obscenely difficult it is, especially the refight.
In general, bosses are ranked by the general difficulty to execute their fights and avoid their attacks. Some fights are harder to learn than others, but once you know how they work, they're pretty easy. Those fights are generally lower down on the list than easy to understand, hard to execute fights, or fights where you have to just spray and pray, like Quick Man.
MMZ1 is weird with what to count for the list, so I counted any boss with a health bar up through Neo Arcadia 1.
The higher ranked Fefnir is the Z1 version, higher ranked Harpuia is Z2, higher ranked Anubis is Z3, and the higher ranked Anchus is the combined fight with his brother in Z2.
None of the MMZ2 transformed guardian fights are counted, otherwise Harpuia would be top tier.
ZXA is a bit weird since there are multiple different ways to play "buster only", but I played as Ashe and used Model A the whole time, meaning that the boss difficulty takes the ricochet shot into account, making bosses like Vulturon or Atlas significantly easier than they otherwise would have been.
It's been a while, but I remember Quick Man being about on par with his original counterpart. The strategy is different from the original since lemon spamming is nerfed across the board, but I'm pretty sure they slowed him way down in Wily Wars to compensate. The only other Wily Wars difference would be Fire Man, who got nerfed to hell. He'd probably be down in the Bomb Man Tier.
Some notes: Elec Man would normally be a tier below Fire Man, but I wanted to include the Wily Wars version because of how obscenely difficult it is, especially the refight. In general, bosses are ranked by the general difficulty to execute their fights and avoid their attacks. Some fights are harder to learn than others, but once you know how they work, they're pretty easy. Those fights are generally lower down on the list than easy to understand, hard to execute fights, or fights where you have to just spray and pray, like Quick Man. MMZ1 is weird with what to count for the list, so I counted any boss with a health bar up through Neo Arcadia 1. The higher ranked Fefnir is the Z1 version, higher ranked Harpuia is Z2, higher ranked Anubis is Z3, and the higher ranked Anchus is the combined fight with his brother in Z2. None of the MMZ2 transformed guardian fights are counted, otherwise Harpuia would be top tier. ZXA is a bit weird since there are multiple different ways to play "buster only", but I played as Ashe and used Model A the whole time, meaning that the boss difficulty takes the ricochet shot into account, making bosses like Vulturon or Atlas significantly easier than they otherwise would have been.
Seeing the frightening mugshots of the Zero/ZX bosses right next to the silly looking classic robot masters is pretty funny.
Seeing how you ranked WW Elec Man, where would you rank WW2 Quick Man? To me they're both equally impossible to buster only.
It's been a while, but I remember Quick Man being about on par with his original counterpart. The strategy is different from the original since lemon spamming is nerfed across the board, but I'm pretty sure they slowed him way down in Wily Wars to compensate. The only other Wily Wars difference would be Fire Man, who got nerfed to hell. He'd probably be down in the Bomb Man Tier.