If we're talking overrated I'd have to agree with Aphex Twin, Biosphere, FSOL.
Those were also the first names that came to my mind. I'm also finding that Harold Budd is starting to wear pretty thin with me. I love some of his releases (e.g., The White Arcades), but he hasn't wowed me in quite a while.
Also, I won't single out individual artists (there are so many now!), but on the whole this new quasi-subgenre of what I can only think to call "hipster ambient" really underwhelms the hell out of me. Adding glitches or layers of lo-fi crackle to lazy guitar drones doesn't magically make them nostalgic or otherwise emotionally resonant. There are some gems here, of course, but they're getting harder and harder to find as the ambient market (such as it is) gets increasingly saturated with this stuff.