I'm personally a big fan of roulette and slots and I look for casinos that allow me to use relatively small bet sizes (around 0.001 BTC).
If there's a progressive jackpot, that's also a big plus for me, especially if they don't require huge bets to be able to win it.
So let me hear what you are looking for in online Bitcoin casino's...
I would say roulette. It's an easy game and I like to play it from time to time. Although I don't do it very often.
I would also look for promotions to help get the house edge a little smaller. But the most important would be the level of trust o the site, and the quality of the support, because most casinos have the same games, so it all comes down to trust in the end.