(National League choices can be found here):
1) Mike Trout (Angels) - 4.6 bWAR, 4.8 fWAR, 4.3 WARP, .341/.397/.562, 168 OPS+
Trout didn't debut with the Angels this season til April 28th, as they felt he might need more seasoning. Since his call up, he's been the best player in baseball. Not bad for a 20-year-old. He leads the league in stolen bases, with 26, and looks to become the first player to win the batting title and stolen base crown since Ichiro's incredible 2001 debut. He also made the catch of the year, so there's that.
2) Robinson Cano (Yankees) - 4.2 bWAR, 4.3 fWAR, 3.3 WARP, .313/.374/.578, 151 OPS+
Cano has slowly but surely become the Yankees best player, and he might be having the best season by any Yankee in the last five years. He leads the A.L. in extra-base hits and totals bases has ended (for now at least) the debate between he and Dustin Pedroia for best 2B in baseball.
3) Josh Hamilton (Rangers) - 3.2 bWAR, 3.8 fWAR, 3.3 WARP, .308/.380/.635, 161 OPS+
Hamilton's incredible production during early May somewhat masks the fact that he's been only mildly above an average hitter at his position for the two months since then (.243/.328/.469). Still, he's entirely capable of another tremendous stretch, and his overall numbers are strong.
4) Austin Jackson (Tigers) - 3.5 bWAR, 4.0 fWAR, 3.3 WARP, .332/.408/.545, 158 OPS+
At 25 years old, Jackson looks to be entering his prime now, as he's cut down on his strikeouts while upping his walk rate and hitting for more power than ever before, all while playing a strong CF. If Jackson hadn't missed 21 games with an abdominal injury, he might sit atop this list.
5) Miguel Cabrera (Tigers) - 3.1 bWAR, 3.2 fWAR, 2.6 WARP, .324/.382/.557, 152 OPS+
Many expected Cabrera's move across the diamond to play 3B after four years at 1B to be a disaster, but he's managed to do a pretty decent job, fighting the position to something of a draw anyway. And of course he continues to be one of the best hitters in baseball.