If you did a survey of Christians and asked them, what is the most important ministry, I am sure you would get a lot of different responses because people have so many misconceptions about what ministry really is. When you look at how the Bible defines ministry, the term ministry is really referring to serving. If we understand this it can change not only how we define what ministry actually is but also how we view "ministry". When we "minister" to someone in some way we serve them and in the context of Christianity and the Bible this service is really something we do for God to help someone else.
One thing that Christians must understand is that anything we do for God is important and vital. From God's perspective, whatever He directs someone to do is significant, no matter how people may view that thing. Too often we tend to glamorize some elements of ministry, like teaching and preaching and downplay some elements of ministry like the ministry of helps or children's ministry. But the thing is, if there is not a harmony of the different aspects of ministry then even the ministry of teaching and preaching God's Word will not be as effective as it should be.
In 1 Peter 4:10 we see that each one has received a gift from God, so each one has to make use of that gift. If we do not make use of the gift that God has given us, if each one of us does not do what God is leading us to do, then God's plans will never come to pass the way He intended. If you are someone who is not really sure what gift God has given you then I encourage you to continue to grow and develop as a Christian through regular time with God by reading the Bible and talking to God in prayer. It also helps to get involved somewhere in your local church to make use of the gift you have. Being involved in your church can even help you discover what gift you have if you don't know what that is yet, as you get involved God will direct you. So what is the answer to the question of what is the most important ministry? The real answer is that whatever God leads you to do is the most important ministry.