No such thing, Imu probably looks the same at all his executives.
And they all share the same rank, the Elders. Holy Knights are military they send to do the job they personally dont.
It is questionable if they are even Celestial Dragons or just trained unit to protect them, just like CP units.
Enel was the same, he had no #2 either. All his executives shared the same rank.
If you are asking who the 2nd strongest is among WG troops, assuming Imu is the strongest, that's another matter.
It is obviously between Garling and Venus as they are the only prominent swordsmen.
Considering that rank-wise, Elders tower above Holy Knights, their strongest member will logically be stronger than strongest among HKs.