The original cover of Pink Floyds Ummagumma with the Gigi LP. With all the new technology things like that are pretty easy, but for the time it was cool, and well executed.
Rainbow 'Rising' is a cool picture too. The hand coming out of the water holding a rainbow along with all the other little things in there that a person really has to look for to find.
How about Paul McCartney's 'Band on the Run' LP with the stars mixed in the band?
I don't know what I'd say was my favorite though.