I would say it is either median or mode. Both are basically unaffected by outliers. For median, order them smallest to largest. 3,3,4,4,4,5,5,5,5,30. The median is the middle number, or 4.5 because there is an even amount of numbers, and 4.5 is between 4 and 5. The mode is 5 because it occurs the most. I would use the median, but it is completely up to you.
