Mega Man 4/YMMV

Anticlimax Boss: Dr. Cossack. He pilots a goofy-looking claw machine that looks like something straight out of a crane game, and it's so easy to lead and dodge that you practically have to make it hit you. It's very likely that this is intentional, since Cossack is a good man being blackmailed into villainy by Wily.
Breather Boss: Toad Man, since he can be looped into an endless cycle where he does nothing but jump. Pharaoh Man too if you use his weakness (he becomes a sitting duck), and Bright Man if you know how to stop him from using Flash Stopper (though he's still more challenging than the former two).
Contested Sequel: While considered to be a great game all-around, whether it's the best Mega Man game is up for debate. To some it's the absolute peak of the franchise, while others believe that it didn't do enough to innovate.
Ensemble Darkhorse: This game's set of Robot Masters is very popular in general. To name specific examples, there's Toad Man for having an oddly cute and lovable design as well as a useful weapon. Pharaoh Man is also massively popular due to looking cool and having a boss fight that hits the sweet spot between "challenging" and "fun".
Even Better Sequel: Thanks to its solid difficulty curve, well-balanced selection of weapons, great soundtrack, and the introduction of the Charge Shot, many fans consider this to be one of the best, if not the best game in the series. Even on the story front, it manages to feel a bit complex and fresh since Wily blackmailing Cossack from behind the scenes was a genuinely surprising plot twist and helped flesh out the game thanks to adding a second Mad Scientist Evil Lair to the mix.
Fanon: He's a Russian scientist whose lab looks like a copy of St. Basil's Cathedral. Naturally, fans like to go all-in with the Russian-stereotypes when characterizing him and his daughter in fanart and comics.
Good Bad Bugs: Tako Trash, the boss of the second Wily stage can turn into a Zero Effort Boss if you know that the Drill Bomb destroys his hitboxes, and thus his Collision Damage. You can walk right through him and shoot him to death from behind while he's completely unable to hurt you.
- This game also marks the debut of the Eddie glitch, where you can re-roll Eddie's randomized powerup gift by leaving and re-entering his room until he drops the one you want.
It Was His Sled: Wily's the actual villain?! Say it ain't so!
Retroactive Recognition: Guess who designed Dust Man? A pre-One-Punch Man Yusuke Murata, that's who!
That One Boss: Bright Man and Dive Man. If you don't know how to manipulate Bright Man's health to stop him from firing the Flash Stopper, he can randomly freeze you in place and body slam you with no way to fight back against him. Dive Man, on the other hand, is harder if you fight him with his weakness because it's a melee weapon, and he is hell to fight up close thanks to his fast-travel homing missiles and Psycho Crusher attack that he can stop on a dime.