Here’s my vote for the best illusion ever. It was created by B. Pinna, G.J. Brelstaff / Vision Research 40 (2000) 2091–2096.
Loom your head toward, and away, from the center point. Print it out and bring to a bar, and it is even more impressive -- the paper appears to be twisting in your hands.
Some other time I'll tell you my speculation for why it works. (It is a
Mark Changizi is Director of Human Cognition at 2AI, and the author of books such as The Vision Revolution (Benbella, 2009) and Harnessed: How Language and Music Mimicked Nature and Transformed Ape to Man (Benbella, 2011).
Best Illusion. Ever.