I was going to say Led zepp's III, both sides are grat, but side 2 is wonderful.
I think record companies usually arrange for the "singles" to be usually on side 1/A, and if they become hits, well then you have a "stronger" side 1/A, or at least one that's more played.
I Agree that an album should be completein both sides, but we all know that is really hard to find a perfect album , one in which all songs are killer to you. With that in mind, one thing that i really like and that i have enjoyed about playing records is that it makes me listen to the whole album, not just the singles or the hits, but the whole package, the way it was intended.