Nightmare armor on thief?
The dungeon armors are really meant to be more for skins you transmute onto other stuff.
If they happen to have stats on them that you want, that’s great, but they’re really intended to be for looks.
then where do you get the best armor or the armor you should use
You basically decide what stats you want on your gear based off of your build. It can vary greatly from person to person. Look up some guides and mold your guy from there
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There’s crafted level 80 exotics with all sorts of different stat combos available on the TP. Along with the ones gotten via drops.
There’s exotics bought with karma from temples in orr with some stat combos.
There’s all the different dungeon exotics, more stat combos.
Between those 3 sources you should be able to get just about whatever setup you want. Mix and match if need be.
After you’ve got your stats setup as you like, if you don’t care for the way you look, then you can always get stuff with looks you like and transmute it onto the gear that has your stat setup.