AAA left it deliberately open as far as criteria, which is fine and appropriate for this kind of tourney/ranking/rating. You could define best album as "best collection of hit songs" in which case Michael Jackson's Thriller is probably way up there, or as "best flowing from song-to-song," or any number of other methods.
For me, I chose to think about it in terms of-- "Which albums did I enjoy so much that I always played them all the way through, and never felt the need to skip a song, nor did I really even have a preferred side." To me that is COMPLETE album, and when I rate "best album" that's the way I think of it. As opposed to being a collection of popular hit singles, which is also certainly a way to rank and rate them.
Honestly, Boston-Boston and The Cars The Cars are two of my favorite "complete" albums of all time. Loved every song, album tracks as much as hits, and never felt the need to fast forward or skip anything. If I started on Side B I was fine with that, too.