The Robot Masters of the Mega Man game series are: (Enemy1 -> Enemy2 means the weapon of Enemy1 will be strong against Enemy2. * indicates that enemy is vulnerable to his own weapon.)
Mega Man 1: Bomb Man -> Guts Man -> Cut Man -> Elec Man -> Ice Man -> Fire Man -> Bomb Man.
Mega Man 2: Metal Man* -> Flash Man -> Quick Man -> Metal Man* -> Bubble Man -> Heat Man -> Wood Man -> Air Man -> Crash Man -> Quick Man.
Mega Man 3: Top Man -> Shadow Man -> Magnet Man -> Hard Man -> Top Man, Snake Man -> Gemini Man -> Needle Man, Spark Man -> Magnet Man. (All MM3 enemies are somewhat vulnerable to their own weapons)
Mega Man 4: Toad Man -> Bright Man -> Pharaoh Man -> Ring Man -> Dust Man -> Skull Man -> Dive Man -> Drill Man -> Toad Man.
Mega Man 5: Stone Man -> Charge Man -> Wave Man -> Star Man -> Gravity Man -> Gyro Man -> Crystal Man -> Napalm Man -> Stone Man.
Mega Man 6: Flame Man -> Blizzard Man -> Plant Man -> Tomahawk Man -> Yamato Man -> Knight Man -> Centaur Man -> Wind Man -> Flame Man.
Mega Man 7: Burst Man -> Cloud Man -> Junk Man -> Freeze Man -> Slash Man -> Spring Man -> Shade Man -> Turbo Man -> Burst Man.
Mega Man 8: Search Man -> Astro Man -> Aqua Man -> Sword Man -> Search Man, Grenade Man -> Frost Man -> Tengu Man -> Clown Man -> Grenade Man.
Yes, as the series advanced, it just became an eight-weapon loop. Only MM8 changed it to two four-weapon loops.