Donkey Kong actually can be a hard game at times, when you first play though. It's also the stages needing quick reactions and thinking to avoid dying that make the challenges even better.
Diddy's Kong Quest was always my favourite, then this one just because of how much better it was and how much I loved it as a kid (and still do now), but Donkey Kong 1 is well worthy of the play too.
So many quality characters, amazing sound and music and great games overall.