Each slot machine is configured to have an optimal RTP percentage. The Return to Player percentage means the percentage of money that is played on the game that is returned to the players. If a slot machine has an RTP of 97%, it will keep just 3%, giving the rest back to the players in the form of prize money. The best paying slot machines or the ones with the highest RTP include:
Ooh Aah Dracula by Barcrest: 99%
Tropic Reels by Playtech: 99%
Jackpot 6000 by NetEnt: 98.9%
Cinerama by Playtech: 98.4%
Blood Suckers by NetEnt: 97.6%
Big Bad Wolf Quickspin: 97.35%
Of course, there are plenty of other games with similar payouts. Check out other games by Playtech, NetEnt or Quickspin with more or less the same percentages.