As with many things, opinions as to the "best" can be very subjective. It seems as though alot of it depends on what you want out of it, what your looking for etc.
So... my favorites are AOPA, Flying and Plane&Pilot. They all have their own plus and minus', but in the end the important thing for me is what I can gleem from them for my benifit or the benifit of others.
Flying has reduced my yearly subscription to 14.95/year, so you can't beat that, Plane&Pilot is about the same and AOPA is included in my yearly membership.
There are other "good" ones out there too, but there are only so many hours in the day to read.