Well this is something I actually know something about
I became obsessed with magazines in my early teens, and started collecting US and UK Vogue, I have every issue of both since March 1983! The collection is so big, we don't have room for it, and most now live in MIL's cellar. They are all catalogued and shelved in order [anal emoticon]
If you are interested in designers, their inspiration and what's realy happening in fashion, the only one worth reading is American Vogue, it is THE magazine.
I think UK Vogue went off the boil a few years ago, but I couldn't bear to not get each edition on the day it comes out, and pour over it while the dc are at school!
Actually, I rate the fashion in Easy Living quite highly, the editor Susie Forbes, used to be at Vogue, and knows her stuff. It has a good mix of aspirational and inspirational, as does Red, although they are both definately aimed at 30+s.
Elle is great for directional fashion mixed with High St. I don't bother with Instyle, or Marie Claire, only buy them if I'm desperate for a magazine.
Harpers Bazaar had a revamp last year, and has improved a lot, but IMO is only worth buying in March & Sept as these are the 'big' fashion issues of the year. Similarly, Grazia is great when the new collections are being shown as they have lots of photos and you can be ahead of the game for next season.