I realize this thread is from quite a while ago, but it came up when on a whim I decided to google "underrated composers" so I made an account just to respond to this. Forgive me if I don't have a hang of the forum interface just yet.
I am honestly pretty surprised to see a lot of these answers, particularly when it comes to overrated. First, it's not completely clear as to what the criteria for "overrated" as it's not obvious if that is meant to suggest they don't deserve to be so highly regarded as a computer or its the poster's personal taste and their music doesn't really do it for them, or even that they are just over played in concerts.
When it comes to considering a composer underrated, I'm not sure if that's the best descriptor. Maybe "underappreciated" would be more accurate, or not as popular as they deserve to be. A lot of the names listed as underrated are very highly regarded and their brilliant art is recognized as truly excellent from an authoritative perspective.
That said, I can hardly believe how often Mozart, Bach, or Beethoven are mentioned as being overrated. When it comes to the impact they had on future composers and their mastery it's not objectively possible to call them overrated. There is not a single composer who came after any of them that was not profoundly influence by their work in every way. All historically significant composers in their shadow knew how great they are and wouldn't hesitate to say so. Despite that, they are certainly overplayed and the real downside to that is how a lot of other music gets overlooked because of it. Still, each one changed the name of the game in their lifetimes. The structure and development of their music is unparalleled, particularly in how they incorporate chromaticism as structural element. Sometimes it's hard to recognize when you don't have the ability to see a world without them.
It's also difficult to truly say a composer is underrated if they still have their work taught in any university classrooms or can be found on the billing of a professional orchestra. It would have to be an obscure genius who no one has ever heard of, but I'm sure they are out there.
I'm through with my rant. Sorry if it comes across as self indulgent. One other point I want to make is I understand the fatigue that sets in from hearing particularly iconic music played over and over again. That shouldn't count as a strike against a composer though. Also, Vivaldi wrote a ton of opera, as did Strauss and I adore his Metamorphosen.