I made my mind up for this one when I was deciding who to vote for as the best thread starter.
Prince Lex became our Site Director this year, and has stepped up to the plate. In 2013 alone, he has made threads like "I think it's time for an Upgrade", "Recruiting Site Staff", "#TLS Being Spammed", "FFVII in a Day", "Filling the Gaps in Content/ Content Goals", a bunch of threads about the community playthrough, and the all-encompassing "Our Site", which has been the single biggest home of site improvement discussion this year. Many of his other threads have been about interesting bits of Final Fantasy news, which is important for our forum.
Lex has also made the second-most front page posts this year (after me, but even if I wanted to vote for myself, I still think Lex has done more for TLS overall), and he started helping out with Weekly Roundups when they were getting too much for me to handle every week. Finally, Lex has been one of the most active people in streaming things, Skyping with people, and generally organising "extra-curricular" stuff for our members.
Dark & Divine deserves an honourable mention for taking over responsibility for Weekly Roundups recently, Fangu does a lot of work behind the scenes that isn't always immediately visible and also wrote her first front page article this year, and honestly, everyone on the list has contributed to the site in some important way this year, but I think Lex has done the most, so I'm very happy to give him my vote