What does it take to be a great missionary? I think there are different types of missionaries and there are different ways they can be seen as good.

1. Innovator. Barnabas. Some missionaries do something that is highly innovative and as such establish patterns that guide missionaries long after them. Barnabas appears to be a great example of this with the strategy that he used for mentoring and then entering strategic locations for missions. Other could include Zeigenbald, and John Nevius.
2. Theologian Paul. Some missionaries develop and (often) write theological works. Such works can be theology of mission, or missional theology. Paul, while certainly a very good missionary as shown in his 3 mission trips. However, what made him great was his writings— 13 letters to various churches. These letters were practical, pastoral, and personal. However, they were very much theological. Other missionaries also embraced theological writing might include Roland Allen and Leslie Newbigin.
3. Promoter. David Livingstone. This is a bit more dubious. Many ask the question of whether David Livingstone was a great missionary. And in his work in Africa they may have a point. At the same time, Livingstone was great in inspiring people to support missions or give more to missions. An even more extreme case may be Henry Stanley, who perhaps could be described as a bad missionary (but a great promoter of missions). Other missionaries may be clearly be good missionaries but are still more recognized in their role of promoting missions. Some examples might include Lottie Moon, Albert Schweitzer, Luther Rice, and Amy Carmichael.
4. Contextualizers. Ulfilas. Some successfully brought the Christian faith successfully into a new culture. Often this is most clearly visible in terms of translation of Bible and liturgy. There are some that are uncertain of Ulfilas because of his theology. No one, however, could fail to recognize his accomplishment in translating the Bible into Gothic language. Others might include Methodius, Cyril, and Ola Hanson.
5. Trailblazers. Adoniram and Ann Judson. Some missionaries may not be innovators in the strictest sense, but trailblaze a new place of ministry— opening the door wide for follow-on missionaries to follow. The Judsons were the first to work in Burma, and their challenging work opened the door for others after. Sometimes they gain the title of “Apostle of _______.” Samuel Zwerner and Nicholas Kassachin are a couple of examples.
6. Organizer. Ludwig Von Zinzendorf. Some may have not done much personally in terms of missions, but they created structures that were highly effective in missions. Von Zinzendorf wasn’t really a missionary, although he did some visits to missionaries in the field. However, he brought structure to the United Brethren (Moravians) creating the first major mission movement in Protestantism. Another would be Hudson Taylor.
7. Faithful. Justinian Von Welz. In most vocations there are superstars. Nothing wrong with that. However, it would be ill-advised to assume that the greatest missionaries were necessarily the ones identified as great in our eyes. Von Welz was an innovator, but too ahead of his time to be recognized as such by most. By almost every measure he was a failure. However, he was faithful even to death. By pretty much every measure William Borden was a failure— rejecting family fortune only to die before reaching his final mission destination. But he was faithful to the call and faithful to the end. These were the Faithful Servant of the parable of Jesus.

There are other categories of greatness. However, I hope you see from the #7 that list in general is a bit dubious. I think it is good as a reminder that there is more than one way to be seen as great or effective. However, our ideas of greatness may NOT coincide with what is God’s view of greatness.