It's the same Greek word used in v15. That's his point. God's grace is great because he is the "foremost" sinner.
15 It is a trustworthy statement, deserving full acceptance, that Christ Jesus came into the world to save sinners, among whom I am foremost of all. 16 Yet for this reason I found mercy, so that in me as the foremost, Jesus Christ might demonstrate His perfect patience as an example for those who would believe in Him for eternal life.
Hi and CHIEF is not there as PROTOS means FIRST !!
dan p