On reddit this was always bait for people who don't know what "mainstream" or "underrated" means. Like I don't know if you've heard of this obscure niche ragtag band... Supertramp? Compare Andrew Garfield's Spider-Man treating every cultural touchstone from prior to 2000 as lost knowledge found in ancient manuscripts.
So I'm gonna say Kingston Wall, a Finnish progressive rock band, and risk finding out that they were a household name for the dozens of people who gave a shit about prog in the 90s. "We Cannot Move" sounds like the pinnacle of 70s production. It just came out while Yes was doing synth-pop and Pink Floyd was doing a Moody Blues album. If it turns out everyone who's heard of Porcupine Tree already knows about them... good.
If we're talking mass appeal that's missing, I'd say The Cinematics. Two and a half albums of energetic alternative rock. Broadly anti-consumerist messages throughout. Guest work from We Are Scientists and Beck. Broke up for unspecified reasons. They deserve to be one of those "oh yeah, those guys" kind of bands. Not mindblowing... but remembered.