Sleepychimp
2001
This game just isn't fun, the level design is a complete mess that feels like it was thrown together in a few hours, the Nightmare system doesn't add anything to the game other than frustration and straight up doesn't work correctly, and the bosses are either unfair or complete pushovers. The only stage that's somewhat enjoyable is Commander Yammark's but even that has leapos of faith over instakill spikes, and Metal Shark player is the only fun boss. Other than that, the only positive thing I can say about this game is that it isn't X7.
1997
This is my personal favorite game in the X series due to the amazing graphics, stage and boss design, and inclusion of Zero, but it has it's fair share of faults. Particularly the special weapons aren't very good and the bosses are complete pushovers with their weakness. Also the story is a mess but it's a funny mess. That said, this game is simple, fast paced fun and one of my favorite games ever made.
1993
A great evolution of the classic Mega Man formula, it's fast paced, has really fun special weapons, and encourages exploration with fun upgrades. There aren't any bad stages and honestly the only complaint I have about the game is that certain bosses are way too easy to kill with their weakness, but even then that's a nitpick. It's the closest thing I've seen to a "perfect" game.