Which book of the New Testament would you choose to do a "discipleship" bible study class on?

[Minister Monardo]

We are nominating a book to study from chapter to chapter in a new discipleship class in my local church. We can choose any book. Which one would be ideal to pick to focus on discipleship or perhaps covering a range of topics on discipleship?

Bonus question - what is discipleship? Which book is best to learn this from in new testament?

The word 'disciples' appears an approx equal number of time in Matthew and John.
My emphasis is on the "red letter" content of both. I prefer to begin in John for
similar reasons as stated above by others, but especially chapters 13-17,
from the washing of feet, to the bearing of fruit...and particularly the preparation
for receiving the Holy Spirit and the Lord's amazing prayer to His Father. in ch. 17.
Matthew 5-7, also mentioned often above can follow immediately, or
interspersed with John.
Matthew 18 is so critical to understanding the role of the Church in
fulfilling the Gospel, and the apostolic doctrines puts the same
emphasis on fellowship among the brethren beloved, so as to
understand Christ's repeated exhortation "to abide", repeated in 1 John.