for me it's got to be Time 1 by Wintersun
i spent £20 on an album simply because i liked the cover art.
it was to be one of my favourite albums of all time.
other honourable mentions:
Smashing Pumpkins - Siamese Dream
Fleetwood Mac - Rumours
Foster the People - Torches
System of a Down - Steal This Album
The Kinks - The Kinks are the Village Green Preservation Society
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Coldplay - Mylo Xyloto only dislike one song on the album.