You have a mesmer and you're trying to figure out which single armor is the best?
Most mesmers have... well... we have a problem with storage due to our wardrobe
It really depends on what you're doing, the + energy set is always nice. the +armor set can be convenient if you end up in meele range a lot + health is nice against degen.. and I have a +armor while enchanted set for soloing.
In reality though, mesmer armor should be picked according to what you like the looks of, none of the stats matter paticularly much.
Runes.. minor fast cast on one piece of armor, vigor on another... and personal preference is to have a head piece with a superior of the same type as the +1 of the headpiece. Also remember, most of us don't run inspiration at too high a level, so a minor inspiration somewhere in your armor will help your build.