Blackjack is extremely popular IMO in most bitcoin casino's offering it, Slots is popular -although I don't know why, odds are awful and I don't find it fun at all-, Dice is popular. Sport betting is popular and is a form of gambling, I think it ranks at the same level or just below dice.
Blackjack are really popular to the people that want to test their skills and also luck. Gambling and are always together.
But dice is the most popular game I know, it is fun because of the risks I think, it is really exciting. If everyone wants to test their luck, dice games are the best.
Sports bettings are also good especially when you are a sports fan. Though I don't really bet on sweating sports like basketball or football, I do bet on tournaments in Dota or LoL, I am a player and I know and understand the plays and it is really exciting.
Dice still is the most popular for me, then sports betting followed by skill based gambling games.