I actually wouldn't claim any of the trashy pop stuff is overrated, a very small minority seem to like it and nobody else. It's the big ones, generally well respected bands that are massively over rated (it seems particularly true of American bands/singers). Being overrated is different from being crap, as such the most overrated bands/singers are the best ones. That said, given how unimaginably horrid rhianna sounds, the fact that her fan count isn't in the negatives suggests she is grossly overrated (and, as anyone with any sense has ascertained, I consider her music to torture in audio form).
Metallica, Guns n Roses, The eagles, Hendrix, elvis, etc.
That's not tot say that there aren't non american bands that aren't insanely overated: the beatles, zeppelin, queen, AC/DC and so on also get way more attention than they should.
That's not to say that they are bad. Quite a few of the listed bands are my favorites, but that doesn't change the fact that everyone seems to love them, and, from an unbaised perspective, they are not as good as their popularity implies.
To throw out something off topic, I feel the cat empire is massively under appreciated.
