Some of the old mega mans were hard as hell. No charge, no dash no nothing. Just a pellet and jump (the great MM2).
In terms of the newer ones, i always have trouble with the black blob guy with the one eye that turns into blocks that you have to remember how to dodge. Not to mention that if you touch him in full form you die instantly.
Sigma in X4 and 5 was kinda hard. Took me a little while.