Well I don't know who is the most overrated but when I hear overrated and I can only think of 2 people:
Britney and Adele:
I always here how Britney at least was good at performing, when I really have yet to see 1 breath taking performance from her .. Her old performances are nothing special at all... but maybe It's just me
And Adele, as amazingly talented she is, people still overrate her and the only reasons for that is the argument "She sings amazing and don't need anything else" and of course the huge sales ..