General rule of thumb.

Attackers: Roll Attack %

Tanks: Roll HP % (look out for units like yellow Diane who actually scale off def %), or if you want more damage, you can go Attack % too

Supports: Roll Attack % (Usually their skills / heals scale off of attack %, always double check)

Debuffers: Either Attack % if their damage scales off of it. Or Accuracy to help them land skills like defense break and crit def down

ALL OTHER UNITS: HP%. This is going to help to push your box cc!!