Currently holding the #1 slot in the iPhone app rankings is Texas Hold’Em.
Apple’s poker simulator is a great time-waster, whether you’re playing the single-player mode or playing against your friends. While the player AI leaves something to be desired (playing tight is frequently enough to guarantee victory, and check-raises work better here than in “live” games), the UI is slick and intuitive and the overall package is superb, particularly for the price point. More advanced players might miss features like play analysis, but most players will thoroughly enjoy this offering from Apple.