I've always considered Led Zeppelin's Presence to be underrated. Especially since it contains maybe their best song in Achilles Last Stand. Bonham really stands out on Presence.
Pink Floyd's Obscured By Clouds is definitely underrated.
Not sure it's underrated but Michael Jackson's Off The Wall is kind of overshadowed by Thriller.
Since there are only five, none of the Police's albums are really underrated, but Outlandos D'amour has several tracks better than Roxanne.
People love to sh*t on Black Sabbath's last two Ozzy albums but if you took both side 1's of Technical Ecstasy and Never Say Die and put them together as an album, that's actually a great record.