The most popular casino I do see is Fortune Jack, you will even see their campaigns everywhere in this bitcointalk forum and they are one of the top Bitcoin Casinos you will find. When you register they give you bonus  up to 5BTC ( seems they also give free spins as well) but you know how the bonus thing works, there is always a requirement you will have to reach before you can have access to withdrawing your bonus, if you don't, then you're not getting it. Check out www.bitcoinfy.net  and you will see a list of the best crypto casinos with reviews and ratings as well.