Eh... Mega Man 8 has Clown Man, and that alone makes it worth not playing, but I also found the super ball mechanic and all the stupid shit you had to do with it to be sort of annoying. That game was probably the one I enjoyed the least. I remember 7 being pretty horrible too.
I think that after 3, the most fun game was probably 6 for all the wacky physics of the rush suits and the things you could do with them. It gave the game a certain amount of versatility. I have mixed feelings on 5 because the super arrow is a fun gimmick, but Rush Coil is horrible and half the weapons are useless. Something about 4 never quite sits right with me even though I enjoy parts of it and kind of liked both the Balloon and the Wire along with most of the weapons. Funny thing though, I found out yesterday that the guy that did the music for 3 also did the music for 4, which was not what I expected. I would have thought 2 and 3 were the same and 4, 5, and 6 were all one guy, but no.
Anyway, I think what makes 2 the most difficult in the series is probably the level design one you get to the Wily stages. The dagron fight wouldn't be awful if not for the position you were in, and similarly, the first security system and the Gutsdozer are no big thing so long as you can get to them (the second security system, admittedly, takes some ingenuity). And the lead up to Wily, where you take damage from the drops without any way of recovering and by the way he's only weak to one weapon? Totally unfair.
100% natural, no antibiotics, and
bloodgrass-fed
