Really surprised BETKING is #1 compare to Primedice. I practically never heard of that site before I looked at your reviews. Its nice that they have an investment option at least.
But what cathegody you have in mind? List can be sorted by parameters like:
total wagered - PD is far ahead of anyone else,
total bets - where PD is far ahead,
opening time - which is not that important as long as site is quite old,
max payout - where BetKing is winning,
total invested - where PD can't be since they don't have investment option avaiable
BetKing is not promoting aggressively on Bitcointalk ATM, nor have very active thread like PD has, so that might be a reason you don't know them.