While nickel slots have the highest payout rate, they're also very rare. Caesars, MGM, Wynn, and other Las Vegas Strip casino operators have largely phased out nickel slots. Only about 1,000 nickel machines are still operating and you will be more likely to find one if you head downtown to Fremont Street.
Other than the rare nickel slot machines (which only exists in old school Las Vegas casinos), the best payout comes from the $5 slot machine, according to the Las Vegas Review-Journal. Casinos had a 5.46% win percentage in 2024.