This is a question that I have long been searching for the answer to. The first, most obvious answer is that it is of a higher quality then almost every other album. While that fact is almost a given, it's most certainly not the actual answer. In fact, I think that while being an integral part of greatness, it amounts to only a small part of the reasoning in the long run. The other answers that seem to be obvious are legacy and influence. Both of these things seem to make a lot of sense in consideration. In fact, they are probably the key to unlock greatness. But the one thing that has a huge baring on both of those things? Time. Merriweather Post Pavilion may or may not be one of the highest quality albums of all time, that's a far more subjective and opinionated topic, but nobody in the right mind could say that it is one of the greatest albums of all time, it's just not right.
Therein lies the difference between "best" and "greatest". The way I look at it, best is completely subjective, but greatness isn't. I think greatness can be measured in influence and importance. How did affect the music scene? Did it start a genre? Did it drastically change a style of music? This is what makes an album great.
Like my "Musical genius" blogs, I'll expand more on this later with specific bands and albums.
Sonic Youth - Daydream Nation The Smiths - The Queen Is Dead The Beatles
Why I believe OK Computer is one of the best, but not one of the greatest albums of all time