This is an interesting question, but I would say that there might not be a definite answer, and instead it depends on the particular people (both the person who needs help and the person giving help).
However, some options seem worse than others:
an ex-romantic partner is probably not the best, as they would not necessarily want to help and the person in need might not have a neutral or positive attitude towards them.
As far as the others are concerned : a religious counselor (of which religion? do they have the relevant experience and training?), older and experienced teen (depends on that person, they might be helpul) and a trusted friend, the last might be the best option.
Not mentioned here, but the best option is general would be a psychologist or therapist.
