Speed.

That will simplify everything.  I have Duchess and FuShan.  Duchess is on most of my teams.  Using her will free up another slot for a champ with buff/debuff that are needed.  

Depends on your team and account.

Pproioritise crit damage on champs and missing he last bit to guarantee crits? get critrate aura.

Speed aura is always solid.

Missing a bit of accuracy- accuracy aura helps.

Clanboss ideally you want defense aura.

most important is your team, whoever then in your team has a useful aura use that one as a lead.

Duchess should be pretty much every team you run in Doom Tower anyway, which makes life simple. :)