What is the greatest ministry we can have?

The greatest ministry is not service. In Is. 6 we have the account of the vision that Uzziah had. He saw the Seraphims in the presence of God, with two wings they flew speaking of service but with four wings they worshipped,  i.e. with two wings they covered their feet sand with two wings they covered their face speaking of worship.
The greatest ministry a child of God can have is worship!