Actually, with the exception of the music, I don't much care for the Classic series; it always seemed very slow and exceedingly difficult to me. The X, Zero and ZX series all had faster, more entertaining gameplay, with the Zero series still maintaining a challenging difficulty level. They all have much better storylines - whilst Sigma is similar to Wily in his... let's say dedication, at least there is a well explained reason for his multitude of reasons, instead of just "Oh no! He's escaped again!"
I'll even argue that the Zero series has better music.
Although if anyone wants to see an interesting take on Mega Man 7 written by one of two good authors on fanfiction.net, or an incredibly well written prequel to the Classic series by the other, I suggest flipping through these;
Mega Man 7
Mega Man Prequel