Aloha,
Personally I only own 3, which are the only three I wanted to grind out, the rest are near completion just from casual play since they were launched over a year ago. But my 2 cents, I LOVE LOVE LOVE the Republique UU, because 19 second main battery reload with a secondary build, makes her a beast. And the DM UU because going backward and forward almost instantaneously can really mess with people trying to shoot at you. The other one I got, but do not use is the one for the Yueyang, I got it before she had her reload nerf, so for my play style doesn't fit anymore, so I no longer use it. Again this is not a "guide" or anything, just what I like.
Mahalo,
-Hapa