I think if we talked about some of our favorite characters, and why we liked them, each of us would cite different reasons, but I also suspect that there may be an underlying principle or two that explain why we like specific characters, in general.
Audiences need to like the protagonist(s), to both relate to and admire them. People often relate to characters that remind them of themselves; characters who face similar types of problems. People often admire characters that demonstrate a skill or proficiency, particularly if that skill demonstrates altruistic motives, such as teaching or medicine, or if it has a strong visual appeal, such as performance arts or athletics. Intelligence is another admirable trait, though when it comes to brains a person can be smart, knowledgeable, or clever.