MM1: Freaking HARD levels. Bosses are pusses, though. Also, scoring system rocks; I wonder why it ever went away...
MM2: More hardness; but manageable. The first real game of "paper rock scissors" that isn't totally obvious the first time.
MM3: This is the one that started it for me; and I still think it's the best. Yes, we know Rush and Blues. But this is also the only game where you replay "broken" levels and face SIXTEEN Robot Masters!
16, in one game. You'd have to drop at least 3 credits to do that, in the arcade versions, which are the only other ones I know of that even feature more than 10. Best master weapons, too.
MM4: Replaced fast-action reflexes with patience. Not the best, but noteable for it's boss levels.
MM5: An overall good execution, but it fails in one critical point. "The Retard Coil."
MM6: A very fun title, and it's good to see them trying something new. Plus, springs and timer-bombs are only hints as to what's to come. FAR too generous with E-Tanks and 1ups, however.
MMV: GameBoy finally gets it's own bosses and master weapons! Fun levels and good music, too. Stardroids and Duo really make me wonder what the first chapter of this saga really is... after all, in MM8, Duo seems to recongize Blues... (I'm not sure if Wily/Light "built" him, at all! Maybe they've all been "Reploids," all along...)
MM7: Amazing title, and probably my 2nd favorite; if nothing else, for the ShadeMan level. It truly went beyond expectations. You encounter Bass there, you encounter and fight Proto there, it has secret music, and the splitting passage at the miniboss - just truly epic. Also, my personal favorite Wily music, ever. Rush Search was fun/funny about the first 2,500 times I used it, too.
MM8: Probably the most playful, fun title; though a bit slow-paced and far too forgiving for my tastes. You know those spots in TenguMan's level, where you ride the bubbles? If that were ANY other game, those spikeballs = death. (See also; WaveMan) And anybody who thinks the FrostMan boarding sequences are hard needs to play the Jet Stingray and Squid Adler levels in the X-series.
MM&B: Collecting the CDs was fun. But outside of that? Was pretty standard faire. Levels are very difficult as MegaMan, bosses are tougher as Bass; which seems pretty fair, I suppose.
RnF WS: Uh-oh! There's an entire set of levels, bosses, and weapons I have *nothing* on! ^.^
MM9: I swear, I'm the last man on Earth (who cares) that doesn't have that game yet. Damned new-gen systems...