IMO, one of the best blackjack cheating moves is/was stacking a double down hand from a new deck.
First, it is a 100% winner.
Second, the agent wins twice as much as his/her original bet. To eliminate confusion the agent bets $1,000 then double downs for another $1,000. Instead of winning $1,000 total; he wins $2000.
Third, the play is psychologically perfect. The agent makes it appear this is a one-time shot, which in reality it is. The agent stands, he never sits down. He plays cash. He appears aggravated that he feels compelled to double down. This act convinces the casino employees this player is taking a one time shot.
Fourth, the move is psychologically perfect. In the old days the boss would open games at the start of Swing shift. He would count down the rack, give the dealer a new deck(s) and move to the next table. This was the perfect time for the dealer to stack the double down hand for his agent. Obviously the boss was not watching and neither was the eye in the sky, because there were no players at the game.
Fifth: The heat will be minimal. The casino believes the player took a shot. The win is not big compared to a cooler and it is much easier to do and less risky.
In the old days when games were opened dealers did not "wash" the cards. They were told to give the deck several shuffles. It was during these shuffles that the stack was created. When my agent came to the table I simply retained the stack during the shuffle and marked the place my agent would cut the deck. The rest was all acting.
Historical Note: I came up with this idea and did it under fire. Others might have done this before I did, but I had never heard of anyone doing this.