Some severely underrated Classic Rock bands are:
J. Geils Band
Mott The Hoople
Grand Funk Railroad
Dwight Twilley Band
Bachman Turner Overdrive
Based on my personal experiences elsewhere on the web and in real life, I'd have to agree with Wishbone Ash - a great and original band which pioneered twin-lead guitars in isolation from the southern rockers yet seem to be largely unknown nowadays even though they inspired a lot of bands like Thin Lizzy and Iron Maiden who are substantially better known. In some ways though I feel Ash has somehow achieved a rare happy medium - the band is respected within the classic rock community but not sufficiently well known outside of it for the music to become overhyped. Argus is a fantastic album and was successful back in the day but has avoided the hyperbole attached to other classics like, say, Led Zeppelin IV or Sgt Pepper.
In terms of respect though, and I am aware that they are better known and more successful than most of the other bands mentioned so far, I'll add Dire Straits. There was a discussion on another board about the band and having stated my liking for Mark Knopfler and co I was repeatedly told that the band's success was largely down to the videos for Money For Nothing and Walk Of Life and that nothing else they did was as good. As a huge fan of the band, my love for them has nothing to do with the videos and everything to do with the music and I feel that anyone who makes this argument probably isn't familiar with anything much beyond Brothers In Arms. I also saw a documentary about Dire Straits which essentially made the same arguments for the videos, and I just can't agree with it. I consider Walk Of Life to be amongst their weakest songs, and there are 8-10 songs that I'd say are definitely superior to Money For Nothing as much as I like it, while by contrast the entire Communique album is pretty much written off by everyone other than the big fans in spite of containing excellent songs like Once Upon A Time In The West and Single Handed Sailor. In summary, what I'm trying to say is that I feel that the Dire Straits deserves more respect for their music and that a couple of their big hits have distorted the perception of their whole career.
I have always thought Great White were out of their time, and wrongfully lumped in with all the hair-metal bands of the '80's.
The Tragically Hip, I just don't understand why they were never huge. Across Canada, and in the US border towns they are still big, but I've heard they were practically unknown in the rest of the US.
I almost took The Faces off my list... they were big and could have been huge... maybe almost as big as the Stones... except for Rod's non-committal treatment of the band. Yeah, his solo thing was big and I can't blame him for not wanting to let that go, but had he focused that energy on the band, The Faces would have been huge. So... maybe not underrated after all; just under-productive.
The Angels (Angel City)
The Tragically Hip
The Georgia Satellites
New England The Faces