it depends on you riding style and interest. I ride XC and I find the Singletrac (UK based) a very useful and informative magazine. it is not geared towards US market, but I think thats a good thing. Kind of refreshing. expensive if you buy from the store. but I still do it on a pretty regular basis.
here is their link
I like Mountain Bike Action. No matter how you fill about it, MBA puts out a lot of practical and useful information. specially for bigginers. I laugh at some of their stuff (e.g. black diamond category)...but, no magazine is perfect.
DartRag....its a good read. mature graphics, writing and content. It does some good reviews but, to me, they dont seem to cover as much as the MBA does. I may be wrong. I take dartrag while I travel or while I am on a vacation.
Finally, I like Bike. they are a bit different. very refreashing. does not pack as much as the MBA or DirtRag. but still a good magazine, if you like something different.
so, my order of preference is (for a bigginer and an XC rider looking for wide range of information)