Personal opinion of course:
Bingo is laughably easy to promote… problem is there’s not that much traffic or keywords to target.
Poker is in my opinion the most difficult to promote and with the exception of Pacific and Party often time has the worst player value for me. I earn much more off the average bingo player than i do off the average poker player and that is not saying much.
Sports Betting is moderately easy to get players but I am not seeing the player value yet.
Casinos have more limited traffic than sportsbetting and poker, but the player value and potential for whales is significantly higher.