Find The Least Common Characters
Mathematica, 102 93:  

v2:

  Grid[SortBy[
             Tally@StringCases[ToLowerCase@#,x_?LetterQ:&gt;x&lt;&gt;":"],
              Last@# &amp;],
      Alignment-&gt;Right]&amp;@s


v1:

TableForm[
         SortBy[Tally@StringCases[ToLowerCase@#, x_?LetterQ :&gt; x &lt;&gt; ":"],
                #[[2]] &amp;], 
         TableAlignments -&gt; Right] &amp;