Lookup the Least Frequently Occurring Value
The following formula returns the least frequently occurring value from A2:A10...
=INDEX(A2:A10,MATCH(MIN(COUNTIF(A2:A10,A2:A10)),COUNTIF(A2:A10,A2:A10),0))
Note that the formula needs to be confirmed with CONTROL+SHIFT+ENTER. If done correctly, Excel will automatically place curly braces {...} around the formula.
Based on the sample data, the formula returns "D".
Sample Workbook: Download