Here is my take on this. Every calling is equal in its importance to the building of the kingdom. Every one has to do his part for this system/organization to work properly and effectively. No one calling is more important than the other. However, each individual called is different. Everyone has their own weaknesses and strengths, and therefore some of us may feel very comfortable to be called to one calling while others may not. I think when someone says they are "humbled" in a calling that means that they felt very inadequate for it and it brought them to their knees more than any other calling they've had before. For me I was "most humbled" in my calling as a primary teacher because teaching kids scared me. While my friends called to the primary with me seemed to have the easiest time I struggled. So of course that was humbling for me as it would be for anyone in my circumstances. Saying that a calling humbles someone, or makes them feel the need to be more humble isn't saying that it is more important than another calling. It simply means that the responsibilities of that calling required one to develop attributes that they were weak in and their for rely on the Lord more.