C

C is the most superfluous letter in the alphabet.  Almost everything it does could be done as well by S or K, with less confusion as to which sound is supposed to be made.  What S and K might not be able to do as well is a "ch" sound, but it would be stupid to keep a letter around solely for its role in a two-letter combo, when instead we could replace C's place in the alphabet with a "ch" character.