I enjoy OCs who feel like real people. Like someone you could have an actual conversation with and get realistic, unique responses from. Consistent personalities are lovely as well. Like if you say your character is shy, don't have them starting every conversation in the story. Or if your character is mature and serious, don't have them act all cheery all the time. Obviously going too far the other way is detrimental as well (as I have personally experienced), but don't say your character is one way, then have them act the complete opposite.