I think justdice, as it is a long-standing gambling website. The only problem is that they use an alt-coin called "clams". These are sometimes considered scam-coins as a few people own the majority of these.
Why do not they use other coins? One reason could be that they are supporter of Clam and Clam is a PoS coin. When user deposit Clams there, Justdice will earn interest with the coins.
Clam is the most held coin in crypt-currency history. Fairly distributed.
Like this many people now a days prefer alt coins to make dice gaming or any other gambling just not willing to risk their bitcoin. This will not letting out bitcoin from gambling industry.
Do you mean Clam has most users? I thought bitcoin or Doge, litecoin have more users. Do you think the site owner have some connections to the Clam community?