Battletoads for the NES is the hardest game I have ever played. I played that game for the better part of an entire summer, and only got to the Grenada Ducts (stage 9, I think) once. Maybe I sucked at it, but I never played it again to see. These guys not only are better at Battletoads than I am, but they felt the need to build a site about the series.
Rare released a slightly easier version of Battletoads for the Super NES a few years after the NES version. I made sure to get this version and beat the $50 I paid out of it. Revenge is nice.