I would pick atvaction or quad personally. atv sport and atv rider sometimes have good info and stories too. I am rather sick of dirtwheels editing....or lack there of...and miss information! The also contradict themselves quite a bit!
QUAD and ATV Sport are the ones the show up at my mailbox. What I like most about QUAD is that it's suited to the "average joe" atv rider. I could care less about half a magazine filled with racing articles. ATV Rider is another one I buy at the newstand every now and again. It is more suited for the trail riders and has some good riding area info.