Name one virtue that, more than any other, leads to happiness, and why?

I believe gratitude leads to happiness. Ingratitude leads to darkness, separation from God, sinfulness.

8 For the wrath of God is revealed from heaven against all ungodliness and unrighteousness of men who suppress the truth in unrighteousness, 19 because that which is known about God is evident within them; for God made it evident to them. 20 For since the creation of the world His invisible attributes, His eternal power and divine nature, have been clearly seen, being understood through what has been made, so that they are without excuse. 21 For even though they knew God, they did not honor Him as God or give thanks, but they became futile in their speculations, and their foolish heart was darkened. 22 Professing to be wise, they became fools, 23 and exchanged the glory of the incorruptible God for an image in the form of corruptible man and of birds and four-footed animals and crawling creatures. (Romans 1:18-23)

Ingratitude is the first sin, weakness, failing, lack of wisdom that leads to the rest. It is the tipping point, where you "trip" into the rest. It is as if gratitude propels you towards God and lack of gratitude propels you away! It is the thing/action/lever you control that makes you move in one direction or another.

Gratitude helps you deal properly with the good and bad in life.

10 But I rejoiced in the Lord greatly, that now at last you have revived your concern for me; indeed, you were concerned before, but you lacked opportunity. 11 Not that I speak from want, for I have learned to be content in whatever circumstances I am. 12 I know how to get along with humble means, and I also know how to live in prosperity; in any and every circumstance I have learned the secret of being filled and going hungry, both of having abundance and suffering need. 13 I can do all things through Him who strengthens me. (Philippians 4:10-13)

Paul was content (happy, satisfied) in prosperity and want. Why? He was grateful for what God provided in every circumstance. His happiness was based on his closeness and confidence in God (continually made possible by his gratitude), not how much he had or did not have. His happiness was not based on how well his life was going or how successful his ministry was… (he was in prison when he wrote Philippians). His happiness was directly related o his personal relationship with God and that relationship was a close one because Paul continually gave thanks for everything in his life: great, small, hard, easy.

So, let us give thanks for this food, this day, this family,… and everything else in our lives, so that through grateful hearts we may grow closer and closer to God who promises us eternal happiness and joy in Christ.

As Paul says in Ephesians 5:15-20:

15 Therefore be careful how you walk, not as unwise men but as wise, 16 making the most of your time, because the days are evil. 17 So then do not be foolish, but understand what the will of the Lord is. 18 And do not get drunk with wine, for that is dissipation, but be filled with the Spirit, 19 speaking to one another in psalms and hymns and spiritual songs, singing and making melody with your heart to the Lord; 20 always giving thanks for all things in the name of our Lord Jesus Christ to God, even the Father;