I have to agree with the previous mention of Boston, in particular "More than a Feeling" but also most of that sadly limited discography (just 4 albums, IIRC). It's even more inspiring given that Mr. Scholtz was a kindred spirit of ours, a real DIY kind of guy. To this day, I get a great big grin if it comes on the radio while I'm cruising in the car; I just crank it up and sing along, yet again oblivious to the finer subtleties of the production. And that's exactly what I'm talking about - tunes so catchy that you don't even realize you're listening to a rare genius at work.
Here's another nominee for underestimated genius: the first King Crimson album.
We forget now just how ground-breaking that collection of songs was. Although it comfortably conformed to classical norms centuries old, it was also fundamentally different from anything that had come before. Sure, it capitalized on new technology (the Mellotron), but the other 95% was old-school instrumentation re-purposed. It wasn't jazz, it wasn't classical, it wasn't rock 'n roll, but an inventive blend of all of them, long before the word "fusion" had been coined to describe the combination of disparate musical traditions. And produced by Greg Lake in such a way as to be simultaneously accessible and novel (e.g. distortion on vocals, a common practice today but not in 1969, or mixed time signatures, previously unknown in rock). A watershed work of creativity that spawned an entire anti-genre that explicitly rejected the concept of genres.