I can't choose just one! I have two favorite albums. They don't contain my favorite songs, but the whole album listening to the songs together are just great. There is the modern album "The Poison" by Bullet for My Valentine https://www.youtube.com/watch?v=1dogt6-nMP4
Terribly difficult decision. Can't even begin to answer it. All of Janis Joplin's creations, Led Zeppelin, some of Evergrey albums, Iced Earth one with the song Dracula on it... Nightwish's first one with the new singer.