Array formula in B2:

=IF(ROWS(B2:$B$2)>SUM(1/COUNTIF(List, List)), "", INDEX(List, MATCH(MAX((LEN(List)&","&ROW(List))*NOT(COUNTIF(B1:$B$1, List))), (LEN(List)&","&ROW(List))*NOT(COUNTIF(B1:$B$1, List)), 0))) + CTRL + SHIFT + ENTER copied down as far as needed.

Named ranges
List (A2:A19)
What is named ranges?

How to customize the formula to your excel spreadsheet
Change the named range. If your unique list starts at F3, change B1:$B$1 in the above array formula to $F$2:F2

Download excel sample file for this tutorial.
Sort text values by length.xls
(Excel 97-2003 Workbook *.xls)

Functions in this article:

MATCH(lookup_value;lookup_array; [match_type]
Returns the relative position of an item in an array that matches a specified value

INDEX(array,row_num,[column_num])
Returns a value or reference of the cell at the intersection of a particular row and column, in a given range

IF(logical_test;[value_if:true];[value_if_false])
Checks whether a condition is met, and returns one value if TRUE, and another value if FALSE

ROW(reference) returns the rownumber of a reference

COUNTIF(range,criteria)
Counts the number of cells within a range that meet the given condition

MAX(number1,[number2],)
Returns the largest value in a set of values. Ignores logical values and text.

NOT(logical)
Changes FALSE to TRUE or TRUE to FALSE

LEN(text)
Returns the number of characters in a text string

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