The mid-late 70's albums are the highest rated among most fans. Most agree that this list includes Dark Side of the Moon, Wish You Were Here, Animals, and The Wall, but there is a great difference in how people rank them.
I personally think The Wall is a fine album, a huge achievement for them, but don't actually listen to it as often as Meddle. This is due to a number of factors, including the fact that a lot of songs from The Wall get/got played to death on the radio, and overall it's just a really long album and I don't often have the time to listen to it in its entirety.
Ummagumma is not for everyone. I like the early spacejam rock stuff and cannot think of four better early Floyd songs to include on the live disc, and I like the individual experimentation stuff too, so overall I like Ummagumma a lot, but I completely understand people who can't get into it.
Atom Heart Mother is similar. It has the sound of a band still finding their feet, but what's really happening is that they're finding a new direction, a new sound, following the exit of Syd Barrett. I like this one, too.
After that, things drop off quite a bit for me. There are some good songs on The Final Cut, A Momentary Lapse of Reason, and The Division Bell, but overall they're just not as consistent and strong as the 70's stuff.
I usually don't like ranking albums, but since you asked, and I'm a pretty big Pink Floyd fan, here you go:
Animals
Wish You Were Here
Dark Side of the Moon
Meddle
The Wall
Ummagumma
Atom Heart Mother
Everything else