When I meant "threat", I meant someone who makes you feel conscious about your weaknesses. You need not necessarily dislike him/her.When you are with him/her, you admire his/her abilities to be skillful in areas which you have tried hard but find it difficult to succeed in. When both of you like the same person, you feel that she has a lot more to offer and is more desirable than you, and there is no way that person will prefer you more than her.
For me, they would be:
Alpha SF: because they seem more outwardly caring and seem to take more initiative than me, and are better in Si-oriented activities.
ESTp: because they have a sunshine-y and dynamic character, are excellent in sports, and know how to groom themselves and dress well to maximise their good points.
ISFj: same reason as Se-ESTp, except that their character is more subdued.