While Daric Barton (OAK) tops the list, Ike Davis (NYM) has endured more strike three calls on pitches located outside of the zone (9) this season as determined by PitchFX. Arizona's Stephen Drew comes in second with 6.
Of course, you must consider volume when reviewing players' ball/strike data. While Daric Barton leads the league in taken called strikes outside of the strike zone, he also ranks twelfth in taken strikes within the strike zone (86), and 2nd overall in all pitches taken (355). So it's not necessarily the case that umpires have been favoring the opposing pitcher over Barton. He simply takes a ton of pitches, increasing the chances of bad calls by umpires. However, other than Barton, only two other players in the top 25 in called strikes out of the strike zone rank in the top 25 in total pitches taken, Carlos Santana (CLE) with 352 and Mark Teixeira (NYY) with 301.