stake.com is an undisputed leader in this matter for me
coinroyale.com is also a good one, but I haven't play there a lot
as for the new ones,
futurobet.com is the one where I spend most of the time when I'm gambling
I will also recommend futurobet it has plenty of games and their holding on to their reputation. Oh you should check out BetHash.io too.
Bethash looks pretty decent, they've got some altcoins available there too. From my recommendations we have: stake, primedice, vegascasino, bitcoinpenguin and futurobet too, they look nice.
Wish that someday there will be a list here which includes altcoins one after one.