Most lists of "the most underrated and under-appreciated" things are a see-through way of trying to boost the list-maker's status by implying how cultured they are compared to the philistines in the audience. But sometimes they do have a legitimate point. In most high school and intro college lit classes, you'll rarely read any Marlowe, but you can be sure to slog through Arthur Miller and Ibsen, even though he's superior to both combined. Someone who complained about the lack of Marlowe in the typical lit curriculum would have a good point, while someone who whined about not teaching Alfred Jarry would just be trying to score obscurity-based status points.
Applying this idea to music albums, let's take Rolling Stone's the 500 greatest albums as the list of what critics hype up. Next, we eliminate the ones that may be good but aren't out-of-this-world great -- that are there just because critics have to revere them or else lose their snob cred. (This eliminates over-hyped ones like The Velvet Underground & Nico's self-titled album). Finally, we eliminate those that the average youngish listener (say under the age of 25 or 30) has not heard much of -- let's say, no more than one song on the album. (This eliminates great but well known ones like Rocket to Russia by the Ramones.) If they've heard two, three, four hit singles but not the rest from it, I'd say they're still fairly aware of that album.
What's left is a pretty good list of "hidden gem" albums. I haven't listened to every one of the 500, so if it doesn't appear, it may be because I don't know it. Here's what I get:
Purple Rain (Prince & The Revolution)
Back In Black (AC/DC)
Raw Power (The Stooges)
Pretenders (The Pretenders)
Electric Warrior (T. Rex)
The Queen Is Dead (The Smiths)
Trans-Europe Express (Kraftwerk)
Psychocandy (The Jesus & Mary Chain)
Strange Days (The Doors)
Honorable mentions (quality isn't quite as high as the others, but still not well known by most young people):
Transformer (Lou Reed)
Siamese Dream (The Smashing Pumpkins)
Of the ones above, the T. Rex album is the best example of a hidden gem, since I'm pretty sure no one under 45 has heard even one song from it and the band is not well known regardless of which albums or songs we're talking about. The other albums would at least be recognized for one song, or for some of the band's other work.