I'd say beating Megaman 8 in a day goes up there, but it is without a doubt the easiest Megaman title in the series.
Beating the original Megaman Zero goes up there too, game was way harder than I expected. Getting through any vehicle parts are usually pretty tough too.
When I was a kid and finally beat all the robot masters in Megman X I felt all accomplished, then Zero died (for the first of many times) and I was super bummed till he gives you that sweet power-up which makes all your abilities level up.
Then I proceeded to fool around with all of the new powers and die an undisclosed amount of times. I have lots of good memories from the original.