For me personally, Meddle was the best. Just love the mellow spaciness of it.

Of course, the whole Pink Floyd catalog is great. Really my favorites stack up like this:

Meddle
Dark Side of the Moon
Animals
Wish You Were Here
Atom Heart Mother
the rest in no particular order

For those fans who are familiar with Dark Side of the Moon, but not Atom Heart Mother and Meddle, I recommend them. As you listen to them, you will notice a clear progression leading to their most popular album.