Shandler's Baseball Forecaster. Spend the $18-20, you'll be happy you did. Some people think that's too expensive, but then they go and buy 2-3 mags that add up to the same amount or more and get far less good analysis than they do in the Forecaster.
If you're going to go with a mag though, the one I like best is "The Fantasy Baseball Guide," as it has some great experts involved (including Shandler and the best player capsules out of any of the mags).
The Rotoworld and Sporting News mags are ok. Fanball is my least favorite so far, their rankings are very suspect and usually when they try to do deeper analysis, they just prove that fantasy football is their true niche.