"I'm still the best hero in this city, aren't I?"

The question tag for "I am" is always "aren't I?" because "am not I" is not used in modern English, and "ain't I" is considered informal.

"You haven't seen my favorite cape anywhere, have you?"

If the main statement is negative ("haven't seen"), the question tag must be positive ("have you").

"We defeated the villain yesterday, didn't we?"

For past simple action verbs ("defeated"), we use the auxiliary verb "did." Since the statement is positive, the tag is negative ("didn't we").