If you join the Mustang Club of America you also get a subscription to Mustang Times (search this forum for discussions about this publication.) Now, if you're asking about general car magazines, I'll put in my $.02 for Autoweek as the best one out there for the general enthusaist.
if you are looking for just a mustang mag then Mustang Monthly or Mustangs and Fords are both pretty good. Like others have said they both are owned by the same publishing group so they are pretty similar in content and such. I usually pick one or both up at the news stand if there is something of particular interest that month. the only magazine i actually subscribe to is 'Musclecar Motion' it has info on all musclecar tastes and is fairly informative on a wide range of cars. with the demise of Muscle Car Review it's readership has picked up quite a bit. it's kinda filling the void left and doing a fine job at it. you can check it out at www.musclemotion.com and www.musclecarnationals.com