MM1's problem is that it's old and jank, 7's problem is that it's weird and not quite a mega man game, and 8's problem is that it's lame as hell. Having now beat MN9, I can say with some amount of certainty that all 3 had better level design.
Look, X5 has some big issues (Mostly in the form of backtracking being forced cough cough Duff McWhalen), but boss and level design is fine. X6 and X7? Yeah, those are worse than MN9. X8 is totally fine though, it's the only one after X4 that's actually good.
I have not played any Battle Network games after the first one, so I won't comment.
Seriously? Legends 1 has stiff movement and bad voice acting, but the rest of it is really good. The exploration of the caves is great, and I certainly had way more fun with it than MN9, I'll tell you that much.
Haven't played any of the Starforce games so eh. Why are you even including the RPGs though? They're nothing alike besides having Mega Man in the title.
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Okay, my turn. Mighty No. 9 has some of the worst level design I've ever seen in a commercially released platformer in ages. Every 'challenge' is either so easy to skip that it's funny or tedious and frustrating instant-death filled garbage, and the mechanics serves to complement that tedium (Seriously, the fact that you can be killed by spikes during your post-hit invincibility is staggering, and hasn't been a problem since
the first fucking Mega Man game). So many stage hazards could have been cool if they weren't just instant death traps, not only is that annoying as hell, it's also just
boring. This game is nearing Metal Shark Player levels of bullshit in stages, and when it's not, it's Infinity Majinion levels of "you're going to get hit here, just live with it, fuck you" enemy placement. As you can probably tell, this game reminds me the most of X6, which is
not good. Not at all. The only reason it's better is because it doesn't have anything nearly as stupid as the Nightmare system and there's no goddamn hostages, but the level design is probably about the same garbage-level quality, for pretty much the same reasons, too, which is really funny, honestly. Funny in a sad way.