Best place to play right now is CoinPoker.  Has quite the range of limits and hundreds of players on at all times so far.

America's cardroom would be second on my list.  It takes 3-5+ days usually to cashout though and you never know what bitcoin rate you're getting.

Nitrogen Sports probably third.  SWC next, but would be higher if it had more players.

BetKing is new and I haven't seen any games whatsoever, so I'm not sure why it's being recommended.  It could turn out to be good though if it picks up, but noone knows.