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.
While there are characters in the Dragon Ball universe - like Zeno, Beerus, and Whis - who are definitely strong enough to take on Goku, Goku also has the advantage of being the main character. No matter how big the challenge, Goku will find a way to overcome it. Even if he's temporarily overcome, the story will ultimately place him on top.
The same can't be said for Chi-Chi, Goku's wife. Proficient in martial arts herself, she isn't the least bit afraid of her husband. Sure, Goku is technically stronger than she is, but there's no way he's going to actually unleash his full strength on her. She doesn't even have to use her fists - she can simply yell at him and be able to win any conflict.
To defeat Goku, all Ryuk would have to do is be able to write his name down in his mystical notebook. He's not the only character from his anime who could do that, but he's the most likely to succeed. Light might get blasted with a ki assault mid-scribble and, if that happened, his flimsy human body would definitely perish. What's more, he wouldn't necessarily know which name - Goku or Kakarot - was appropriate.
Ryuk would be able to figure it out just by looking at him. And since he's an immortal god, Goku wouldn't be able to affect him. He's also a better candidate than Rem - Rem might actually think a hero like Goku deserved to live, but Ryuk would dispatch of him quickly if he thought the ensuing chaos would be entertaining.
The only way that Ryuk could fail is if his magical notebook doesn't actually work on Saiyans - but since the series has no rules about sentient species from other planets, it's safe to assume that it does. Sorry, Goku, but even you can't stand up to the destabilizing power of a cardiac episode.
Goku might be able to survive planet-ending energy blasts, but Saitama can wipe out an astroid with his bare fists. In fact, one of Saitama's most basic personality traits is his ability to eliminate everything he touches by punching it. Goku, being part of "everything" is no exception.
There is a slight chance that Goku could move quickly enough that Saitama wouldn't be able to land a hit, but all he has to do is land one, and it's over for Goku. Meanwhile, most of Goku's fights involve multiple blows before anybody can be declared a winner.
The Truth is an immortal being that no one can stand up against - including Goku. With no effort whatsoever, the Truth could disassemble Goku's entire body, force far more information into his brain than he is mentally or emotionally prepared to handle, and utterly wipe him out.
In addition, the Truth doesn't really have a physical form, so none of Goku's attacks would have any impact on him whatsoever - all they'd do is drain Goku's stamina. His best shot at defeating the Truth is to stay away from him - which is fairly easy if he doesn't take up alchemy.