Personally, I love Onatel’s design and think she is very dart-worthy. I’ve written a lot about her in this thread: Onatel, best hero idea
Justice is a tanky tank, but that’s all she does. Very low attack stat makes her nearly useless against titans or on offensive raids/wars.
Leo would also be a solid choice. His attack has gotten much more respectable over several buffs in 2018.
So my vote would be Onatel, but with a nod to Leo.
EDIT gender pronoun corrections.