Aura is the leadership skill of your champion when you choose him as a leader in battle. Choosing Aura skill is heavily depend on your team and enemy team. like if you are using lots of Defences hero like Martyr or Tayrel then a defensive aura is great. and if you need spd on all champion then spd Aura.
Best Aura For Campaign
For Campaign, ATK and SPD aura is Good
Best Aura For Arena
For Arena, SPD Aura is perfect Choice.
Best Aura For Dungeons
For Dungeons, you can go with ATK or SPD if you have enough HP and DEF so that you can clear stages 100% on Auto. or else you can go with DEF and HP but the battle will take a long time to finish.
Best Aura for ClanBoss
For ClanBoss Speed is not important. You need DEF and HP to survive the damage.
Note – these are the Ideal Aura and as I said its mainly depend on your team. So first look at your team then choose the Aura skill.