I think the best invest site will be Betking.Io...why?
You can check here at Dicesites.com.Their profit is better than any other bitcoin dice site who have invest option.Or you could convert your bitcoin to XMR and invest on Safedice.
Betking could have the best profiting dice site currently but you should consider the total invested amount with kelly and the amount you are willing to invest too. If you will only own around .001% of the bankroll i don't think you would get much profit from it but if you invested your money in a lower bankroll casino you would feel the profit much better