PrimeDice is definitely the most popular dice site. But I don't know how people define "Trusted".

I have always seen trust as how many people trust a site with their Bitcoin.

I am very sure that https://betking.io holds on to more Bitcoin of player and investor funds than any other Bitcoin casino/dice site as we hold close to 5000 Bitcoin.

Now PrimeDice might have more than 5k Bitcoin but that is their own money and not players.

So BetKing is trusted by people to secure 5k Bitcoin so I believe that means it is the most trusted site, but it is not the most popular.