I think Dr. Wily's Revenge (i.e. the first GB Mega Man/Rockman World) is the only one in the series I've ever legitimately beaten (and as a 7 year old, at that). It's also the only one I owned, so that may play a part in it.That was the hardest one, in my humble opinion. It's also seriously underrated. It's lacking in content, sure, but it's got some of the best stage design in the series.