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.
In case my posting history (and in fact user name lol) doesn't make it obvious - Infected Mushroom
It's impossible to pick a single example or a favourite track, since they vary so widely in style and it depends on what mood your in! But some of the better known classics are (best heard on headphones, unless you have a really incredible audio system):
Sleep Token. They're not as obscure as they used to be. They kind of blew up this year but only in the metal scene. Not quite mainstream yet but I think they'll get there.
Mutemath is one of my favorites. Every album is different. In recent years they got more electronic sounding and the band broke up a couple of years ago.
Cari Cari (just discovered them, psychedelic jams)
Bargain Music (reggae/ska, similar to sublime)
Secret Chiefs 3 (schitzo alternative/middle eastern, many members of Mr. Bungle)
TR/ST (darkwave)
The Two Man Gentleman Band (old timey barbershop banjo)
Superorganism (psychedelic)
Me First and the Gimme Gimmes (punk covers)
St. Vincent (indie/alternative)
Poets Of The Fall. They aren't too known but they are the ones behind many Remedy games soundtracks including Control and Alan Wake. Dam they're good, especially their Alexander Theatre session and live performances.
LūN (EDM)/Lights (electropop) (same artist, different personas) has some great music if you'll like that kind of stuff. Probably most recognized in Canada, much less so in the US and rest of the world.
I also enjoy Echo Movement (reggae) but I'm not sure if they're still making music or not.
Here's a few sludgy/doomy metal bands I've been listening to a lot lately: Froglord, Squalus, GUHTS, Grayceon, Hippie Death Cult
Also I think he's starting to breach mainstream popularity, but I'd be remiss if I didn't spread the word of Amigo the Devil (dark country/folk, "murderfolk")
They're not really considered underground since they have 1 million monthly listeners, but I still don't see a lot of people talking about the band Foreign Air. They are known for their hit "Free Animal", but they have so many good songs. I recommend "Your Touch", "Hello Sunshine Pt 2", "Wake Me Up", "Moving On," and "Monsters". Unsure of how to describe their genre, it certainly isn't rock nor bubbly pop...
I love the Delgados. They broke up a long time ago and cited being dissatisfied with not obtaining much success. The most exposure they saw was one of their songs being used for an anime opening. Alun Woodward released one pretty neat solo album as "Lord Cut Glass", and Emma Pollack made (is still making?) a few solo albums afterwards that are all pretty good, too. I think their drummer was one of the most unrecognized musicians working in popular music ever. The drums on the Delgados albums go hard.
I was miffed when, in the early 90s, grunge swooped in and wrecked powerpop's resurgence. MTV had Jellyfish in every corner, and seemingly overnight Nirvana took over.
In the search for harmonies, major key power chords, and jangly, summery guitars (not to mention hummable hooks galore) I had to dig.
The Well Wishers
Dressy Bessy
The Lolas
The Beths
The Dahlmanns
The Riffbackers
And I know I'm missing so many more feel-good bands...
Really obscure: I like Ramsay Midwood, Have Gun Will Travel, and Joshua Ray Walker too, so good and a class act as a person.
Somewhat known: I love Ondara, Durand Jones & the Indications and all their associated solo stuff, Charley Crockett, Toro y Moi, Phosphorescent, Josh Ritter, Arlo Parks.
Oh my God how could I forget, they are broken up now but J RODDY WALSTON AND THE BUSINESS! They are one of the best live acts I've seen and their records were also great.
I like Will Wood, his stuff has been described as "evil jazz", but it ranges from calm and soothing singer-songwriter type stuff to a full upbeat jazz band with chaotic composition and arrangement. I find his lyrics insightful, but they are also often quite abstract.
Cute girls doing cute things have some nice tracks, but many of them feel a bit the same-y.
The Vienna Clarinet Collection has great covers of classic clarinet songs, but classical and jazz.
I couldn't choose one, so here are a few jazz bands I like: Francis Coppieters, Loftys Comet, T-Square, Micheal Lowerstern, Archie Shepp, Sons of Kemet, and Mansur Brown
Pretty much my whole genre is obscure - appalachian "old time" fiddle music.
The best way to experience this genre - sitting the middle of a jam, preferably with an instrument. Here's boys them buzzards are flying from that perspective.
Some more examples I like:
Here's Dan Gellert being funky on fretless banjo. Tune is black eyed susie.
Here's his daughter Rayna Gellert & co on red steer