As you might guess by my sig and avatar, my personal opinion would be that Betcoin.ag is the best site for bitcoin poker. For me personally, it is the best for a number of reasons. The main one being that this site offers me and all other players the highest possible effective rakeback out of all the other bitcoin poker sites, even higher than regular fiat poker sites, in fact.
One can earn well over 150+% effective rakeback at betcoin.ag. Contributing to one's effective rakeback at betcoin.ag are: VIP program, table starter bonus, deposit bonus, daily surprise box, tournament ticket gifts, referral/affiliate program and very generous sig and av campaign. I use all these opportunities that betcoin.ag gladly offers to increase my effective rakeback.
Other big reasons:
Access to the extensive WPN tournament pool.
Also, betcoin.ag's poker client supports HEM2.
Sure, they aren't without their warts and blemishes as there have been issues in the past and some people still have issues with them today, but for me and my poker needs, betcoin.ag is the best hands down.