The most useful sleight, in my opinion, is the jogshuffle. I guess it dependon you style. I'm a big admirer of Harry Lorayne and Paul Gordon. Both of these artists are very creative when it comes to exploiting the jogshuffle.
The Double Lift. I dare someone to prove me wrong.
Unless you count misdirection as a sleight.
The goal of a double if to flip two cards exactly as you would flip one card. If the double has achieved this goal, you wouldn't be able to tell, no matter if you were a magician or a laymen. Obviously, whoever you're watching needs to work on their double which is no surprise, because nearly everyone needs to work on their double so that it looks EXACTLY the same as the flipping of a single card in every way, shape and form imaginable. This includes both technical skill and body language. There are very few people in the entire magic fraternity that can execute a perfect double (with the one single exception that I know of being Benjamin Earl).But any magician, even a beginner, can always tell when a double lift has been performed.
The goal of a double if to flip two cards exactly as you would flip one card. If the double has achieved this goal, you wouldn't be able to tell, no matter if you were a magician or a laymen. Obviously, whoever you're watching needs to work on their double which is no surprise, because nearly everyone needs to work on their double so that it looks EXACTLY the same as the flipping of a single card in every way, shape and form imaginable. This includes both technical skill and body language. There are very few people in the entire magic fraternity that can execute a perfect double (with the one single exception that I know of being Benjamin Earl).
The double lift is not the hardest sleight to perform; it is the hardest sleight to perform deceptively.
Some erotic, evil rodeo clowns should self-destruct.Hence: The DL is the best sleight. No cookie for you.
In my opinion, this is really an impossible question. It's like asking Blondes ofrBrunettes (but that's a bad example because we all know Jennifer Morrison > Blondes + Brunettes, but I digress).
As DawnOfMagic said ever so elegantly, every sleight contributes to a different trick in a different way. It's only after you combine all the sleights and make them flow with presentation that you get magic.
Yup, double has to be most important, it's your foundation.
The double lift is not the hardest sleight to perform; it is the hardest sleight to perform deceptively.