To start, we've identified some of the best US online slot casinos. These are Las Atlantis, Boada, Unbent and Bet365. Each offers a unique mix of features and benefits. However, there are some things that all of them have in common. As a rule, the best USA online slot casinos don't host games that don't pay out. For example, many slot machines in the US only pay out around 75% of their total bet.