I had a think about my criteria as I came up with my list, and they had to have these things:
1) it had to be great from beginning-to-end, and have no dull spots (very rare on ANY LP).
2) the songs all had to have really great melodies throughout
3) there has to be a unifying concept of some sort permeating through the album
4) the material has to be played with a high level of energy and spirit and
5) they have to heave their fair share of 'hit singles' or well-known songs spearheading the album
Most of my list resulted in 90s grunge; they had the ingredients of high energy, catchy melodies that could almost pass for nursery rhymes, and very rarely finds a lull or flat spot in proceedings.
If I think of classic albums that have a massive 'cultural impact', it's weird, I end up digging maybe 30% of them, but find there to be a lot of filler. But if you add the Beatles albums into the mix - I find they had such a high level of consistency, they would trump even most of the albums in my top 10 here. That's where I put The Beatles on a higher level to all of the other major bands in rock music history - The Who, The Rolling Stones, The Kinks, The Beach Boys.. all really great singles bands - their 'greatest hits' albums would be right up there with any Beatles album.. but their albums alone can be a bit hit and miss. Even Pet Sounds, Exile on Main Street, Who's Next.. some of it is okay, some of it is great, but there's a lot of murk there too.
What can I say.. I'm an 'all killer, no filler' kinda guy!!