Okay. Controversial opinions time.
I love the classic series way more than any of the X games. The first Mega Man X is a fine game, but I had trouble getting into any of the others (although I've played and beaten up through 3). The bosses in the later X-games seemed a little too silly, and not in a self-referencing humor kind of way, either. "Overdrive Ostrich?" Really?
I adore the classic series for its simple game play, and clever course layouts. Between the insta-death spikes, pits and other things, I always found classic Mega Man to require greater precision over its more battle-focused sequel series, and that appeals far more to me. I have a special place in my heart for Mega Man 4, as it was my first introduction to Mega Man. I remember watching a friend play, and a boss battle being really close, and then there was that explosion like when Mega Man dies so I thought he lost ... but it was the boss who exploded! CRAZY EXCITED LITTLE AEVIN!!!!!
And now, time to lose all credibility: I loved Mega Man 7.