My pick is Red Rocks, for sure the best venue for sound quality I've ever been in.

Any old theater or auditoreum is good because they're built for sound
here in ohio vet's memorial in columbus is acoustically perfect and offers great sound.
blossom music center in cuyahoga falls is a huge bandshell built for the cleveland symphony , the sound is good but I understand they have to do a lot of tweaking because it was built for acoustic music

RCMH has great sound - enless your to far under a balcony - then the sound is muffled a bit.

Chicago Auditorium has fantastic sound...

Saw my first 2 Furthur shows here...