So far I've finished Mega Man 1,2,5 and 6 from the Anniversary Collection. The hardest game so far was the original Mega Man. Some things which made that game so hard were:
1. The controls in the game. It seems like you're on ice no matter what level you are on.
2. Even we you get damaged, you still cannot touch the spikes.
3. Getting damaged apparently allows Mega Man to travel through matter. Can anyone explain why you sink through those helicopter things when damaged? I almost trashed my PS2 at those part on Ice Man's level and Wily's first level.
4. Gravity appears to be about five times larger than in the other games. I don't know how many times I went to drop from a ledge and I was dead before I even had a chance to move my finger.
5. The Shadow Devil was really hard to dodge with the sluggish controls and the inability to slide. I tried to use that menu trick or whatever, but I clearly had no idea how to properly do it.
6. There is no possible way to dodge Fire Man's attack.
7. The enemies are all as irritating as they look. My least favorite are those sadistic looking flying helicopters with the big teeth and of course the helicopters you have to jump on that shoot bullets out of the side of their head.