Primedice.com or Betking.io. Primedice never showed their bankroll but i think they have bigger bankroll than any other site.If you see the stats on dicesites.com most bitcoin wagered on primedice and i just saw their chat 2k+ online.So it's should be easy to think they got a big bankroll.Betking proved/Showed their bankroll in public.
The 2k online users in chat is either a bug or fake as pointed out in their thread.
I don't know if they actually have a bigger bankroll than BetKing. They very well could but I doubt Stunna would keep all his Bitcoin in the PrimeDice bankroll. When hufflepuff stole 2000 Bitcoin off them Stunna spoke of closing the site IIRC and ended up lowering the max payout per bet.
PrimeDice bankroll is certainly big enough though. The BetKing might even be too big.
There's no way any bitcoin business has an average of ~2,000 active users online at the same time. 200 would be a great result.
If I had to speculate, I'd say PD doesn't hold more than 3,000 btc in bankroll, that's assuming max profit of 30btc is set at conservative 1% of BR, there's simply no need of holding more. Not that it matters though, any amount above users' total balance is Stunna's private funds, he could have 10k one day and withdraw half next day and this wouldn't make any difference.