I find these armors useful on FOnewearl depending on the situation:
-Ignition Cloak-
A must-have for episode 4.
-Tempest Cloak-
A bit more situational, but you should still have one lying around.
-Congeal Cloak-
Same here.
-Sacred Cloth-
Underrated armor. Built-in Cure/Paralysis can be pretty handy in certain quests.
-Lieutenant Mantle-
Built-in Trap Vision. Can be very useful in some quests. When playing in a group, tell your android players you're using this since you will detonate their traps with your techs.
-Sweetheart-
Yes, FOney ATP is really bad. However, this can still be nice to have in quests where none of the other armors are needed. Vivienne, for example, can get quite a boost out of it. Also, the heart effect is just really cute.
-Crimson Coat-
Only for locking Vol Opt. FOney cannot use Excalibur, but with this armor a Red Saber will do the job.
-Brightness Circle-
Well, I never use this one, but I still wanted to put it here. Female characters cannot use Dress Plate, sadly, so this is the best EDK a FOney can get out of a Frame.
-Virus Armor: Lafuteria-
Looks super cool and has good DEF. I'm not a fan of Aura Field due to the TP drain.
I'd say that you should probably get an Ignition Cloak first. Sweetheart is really kinda niche on a FOney and the PDs needed for it are better spent on essential items like a Slicer of Fanatic with hit, V-units, S-Ranks, and so on.