I do believe that it is Lucifer.
Amenadiel is indeed God's greatest warrior though.
Lucifer and Amenadiel fought many times, yes, but each time that Amenadiel won Lucifer was holding back (1x07 & 3x11). However, the one time that Lucifer actually fought back he won (1x12).
Even though Lucifer is the strongest, Amenadiel would still be considered to be God's greatest warrior, because Lucifer is not one of God's warrior.
As proven in 3x26, God does care about Lucifer, but Lucifer still considers him to be his enemy, for very good reasons btw.