Goku is known as one of the strongest anime characters of all time. While the Saiyan warrior is certainly powerful enough to be worthy of that distinction, that doesn't mean there aren't anime characters who could beat Goku. Actually, there are a lot of them - and some are pretty surprising.
There are some super-strong anime characters who could take him on - Alucard of Hellsing and Gilgamesh of the Fate series could definitely kick his behind given the right circumstances. Other characters aren't necessarily as good at fighting, but could easily take him down through other means - all Lelouch of Code Geass has to do is stare at him and Goku's done. Other characters, like Haruhi Suzumiya, could simply reset the world so that they were never defeated in the first place.
Goku might be awesome, but he's not infallible. Vote up the characters who you think stand the best chance of defeating him.