IVs

Varies by hero. Universally though I would say Attack just barely edges out speed.

Neither, a bane in agility is the worst.

I would say speed since it affects double hits but it really depends on what unit you are talking about

Depends on the unit. A lot of the times, a bane in speed can be worked around, or used to your advantage. A bane in attack can't necessarily be worked with unless the unit has extremely high attack (Ex. W!Chrom) or you use an Attack+3 Seal to neutralize the bane.