Given how much I love music, and how passionate I am about a number of bands with long resumes, the "best album" question is something of a tough one.
As a Pink Floyd devotee, it'd perhaps be expected I'd go the predictable "Dark Side...", but no. It's a great album, but not my personal favorite.
For me, I'd have to go with "Wish You Were Here" by the Floyd. More atmospheric, mellow, deep and thoughtful (IMO). Also an amazing disk to both relax to, and well....do other things to.