The royal flush is a case of the straight flush. It can be formed 4 ways (one for each suit), giving it a probability of 0.000154% and odds of 649,739 : 1.
Despite countless other variations and new gimmicky formats, No-Limit Hold'em remains the most popular variant of poker, and therefore the most profitable, due to the simple fact that everybody is playing it. The game is incredibly simple to learn, giving it a mass appeal.
