Hands down, the Saavedra position (above, White to Move and win).
Why? The right mix of history, "no computers to solve it back then" romanticism (a win in a previously-thought-of-as-drawn position) and the relatively fewer number of pieces to let even a novice take a stab at solving and appreciating its beauty.
What do you think is the best endgame study or puzzle of all time? Discuss.