So, you two can either imagine Paul is saying he's the worst sinner, or he used to be the worst sinner, but you should look at the next verse where he explains he is the "FIRST" of a pattern to them who would come thereafter believing unto eternal life.
1 Timothy 1:14-16KJV
And the grace of our Lord was exceeding abundant with faith and love which is in Christ Jesus.
This is a faithful saying, and worthy of all acceptation, that Christ Jesus came into the world to save sinners; of whom I am chief.
Howbeit for this cause I obtained mercy, that in me first Jesus Christ might shew forth all longsuffering, for a pattern to them which should hereafter believe on him to life everlasting.