Many wonderful illusions have been mentioned, and it's hard to mention only one. However, for me, it is the Moretti Box. I want back time after time to see performances of in "It's Magic".
Years ago, Jay Marshall mentioned to me that his favorite all time Illusion was the original Blackstone Tire Illusion as done by Harry Sr. Harry Jr did a version, but it was not quite the same method.
Since Pete Biro mentioned some smaller effects, I'll mention my favorite piece of apparatus magic. It's Richiardi's version of the classic Orange, Lemon, Egg and Canary trick. I've watched the video of it countless times, and the choreography is simply marvelous. Reminds me of the routining of Cardini's act. Dimmairre and Tommy Wonder do fine versions, but Richiardi's was the greatest. (My opinion, of course.)
Dennis Loomis www.loomismagic.com