Overrated - Kingdom Hearts, anything Bethesda makes, Heavy Rain/Beyond: Two Souls, Assassin's Creed (all of them) and generic FPS/Sports games (CoD/Fifa and the like).
Underrated - Hitman series, Tales of series, Okami
Don't get me wrong, I love Bethesda's stuff even if they're crap as PS3 ports and I enjoyed Heavy Rain and Assassin's Creed but I think they get way more hype than they should.
However I will never understand the love kingdom Hearts gets, the controls are stiff and the camera is awful.
Totally agree on everything except the Heavy Rain part
I consider it to be one of the rare gems for PS3.