A few already mentioned are high up on my list, Close To The Edge, Reign In Blood, Spirit of Eden, and The Wall really brings a pang of nostalgia from when you bought an album and felt you owned something really special.
But I love The Orb, he takes loops and samples and makes them sound absolutely gorgoeus (I could probably reel off the 'Vincent and the Doctor' speech here with the artists name substituted).
So UFOrb, which is my choice for his most consistently good album all the way through, takes the top spot.