I'm not talking about albums that were released before the band got famous, I'm talking about albums that came out while the band was famous, but seem to have gone into obscurity nowadays.

weezer's Pinkerton album - it had some great songs on it, like "El Scorcho", "The Good Life", "Pink Triangle", "Why Bother?", "Getchoo", "No Other One" and "Tired of Sex", but it was a commercial failure

it had a different sound than their more popular first album; they produced it themselves instead of what was done on the blue album by Rick Ocasek of the Cars

Is it just for bands/groups or can solo artists be included?

Prince's album for the 1989 Batman film hardly ever gets a mention in interviews and not one song is ever included in a "Greatest Hits" compilations.