What to farm from guild shop

After completing Rey, I used the guild shipments to complete a droid caller. But it's slow and expensive. There's plenty more gear I can get from there, but I don't know what is best:

1. Jawa Engineer for crit damage challenges.
2. Sun Fac, Rex or another meta character (probably Sun Fac first).
3. Raid gear for my arena team.

Help me choose, please? :)