I personally love Escher's Hands Drawing Each Other work, and the Endless Stairwell. I forget the names of those peices, but they're the most memorable to me.
And let's not forget the.. ah.. the word eludes me, but the drawings he did of rooms that weren't exactly three dimensional. One of the more famous ones had a series of stairwells and archways (a la the scene in Labrynth), but another that stands out is a "room" with doorways to an outside garden. The details escape me, but it was something that popped into mine when I was describing the "stairs" work.
I found a site of his works:
http://britton.disted.camosun.bc.ca/escher/jbescher.htmHands Drawing Each Other: "Drawing Hands"
Endless Stairwell: "Ascending and Descending"
The stair room: "Relativity", though he had a few similiar ones.
The "garden" room: Also, "Relativity", now that I've gone through them and examined them all.