SCAARN by Steven Daniels
Impromptu freely selected card at any random number
A spectator freely chooses a card from a shuffled or borrowed deck. They choose where that card goes back into the deck and the deck is handed to them immediately for them to shuffle. A spectator then learns a fair, easy way to shuffle the deck, ensuring the impossibility for the magician to find their card. After this, spectators choose a random number in the fairest way possible. Even so, you end in the best possible position. You can hand the deck to any spectator and once they take it you don't have to touch it or any of the cards again. They deal cards face up from the top of the deck, down to the randomly chosen number. You ask what the chosen card was. They turn over the card counted down to and it is the chosen card. It works every time, it's completely impromptu and gets amazing reaction. This is SCAARN: Selected Card At Any Random Number.

SCAARN is impromptu

There is no set up involved at all

The card really is freely chosen

You have no idea what the random number will be until it is chosen

The spectator doesn't tell you what their card is until they are ready to turn over the card counted down to

The spectator gets to turn over the card

You get amazing reactions.