Which word would best describe an individual who often flatters those in positions of power?

The term "sycophant" is the most appropriate descriptor for an individual who frequently flatters those in positions of power. Sycophants are characterized by their excessive compliments and ingratiating behavior aimed at gaining favor or advancing their own interests. This behavior is often viewed negatively, as it implies manipulation and a lack of authenticity in interpersonal relationships.

In contrast, the other terms do not convey the same meaning. A rebel typically opposes authority or challenges the status quo, which is the opposite of flattering those in power. A hero is celebrated for courageous acts or noble qualities and does not specifically relate to flattery or manipulation. A mentor, on the other hand, is someone who guides and supports another, often providing wisdom and advice, which differs significantly from the self-serving nature of sycophantic behavior.