Also question is, has Saitama ever been defeated?
Yes. He has been defeated a ton of times, and it is strongly implied that he loses regularly. Fubuki almost forced him to join the Blizzard group after one of her people pounded him into submission. Only intervention by the strongest man alive, King, saved him.
Subsequently, question is, who is the strongest enemy of Saitama? Boros
Consequently, does Saitama have a weakness?
Saitama has no weakness. We actually dont even know his full strength. But Saitama doesent need oxygen, he doesent even realize that he is in space and when he does he does the normal thing any human being would have done, holding his/her breath.
Can Garou defeat Saitama?
So far only really Awaken Garou and Boros have even slightly challenged Saitama, however, it's rumored that the series is building up to a final throw down with God who is probably the only villain (that we know of) who could give Saitama a decent fight. For now, there is no one who could beat Saitama yet.