Vivaldi and Adams were the only two I voted overrated. Adams probably not as much, but for me personally, Vivaldi's music just blurs together too much for him to be pleasurable to me. It just sounds like cliché italian baroque music to me and that gets old after a while.
I definitely think that Charles-Valentin Alkan is one of the most underrated composers I've ever come across though. I can't express how underrated I feel he is. Same with Schnittke.