The answer is Paul.
John is traditionally credited with writing the Gospel of John, three epistles (1 John, 2 John, and 3 John), and the Book of Revelation. However, he is not known for writing many of the epistles in the New Testament.
Paul, also known as the Apostle Paul, is credited with writing 13 of the 27 books in the New Testament, which are primarily epistles. These include Romans, 1 Corinthians, 2 Corinthians, Galatians, Ephesians, Philippians, Colossians, 1 Thessalonians, 2 Thessalonians, 1 Timothy, 2 Timothy, Titus, and Philemon. His letters were addressed to various early Christian communities and individuals, making him the most prolific epistle writer in the New Testament.
Luke is traditionally credited with writing the Gospel of Luke and the Acts of the Apostles. He is not known for writing any of the epistles in the New Testament.
Bartholomew is one of the twelve apostles, but there are no writings attributed to him in the New Testament. He is not known for writing any epistles.