B C E G K Q S W

The missing letter is "M".  

Arrived at by numbering the alphabet from 1 to 26 by assigning 1 to "A", 2 to "B", 3 to "C", etc.  Each letter in the puzzle corresponds to the prime numbers 2, 3, 5, 7, 11, 17, 19, & 23.  The missing number is 13 which would be the letter "M".