What comes next in the series: Z, W, R, K, ?

Let's look at the positions of the letters in the English alphabet:
* $\text{Z}=26$
* $\text{W}=23$ (Difference: $26-3=23$)
* $\text{R}=18$ (Difference: $23-5=18$)
* $\text{K}=11$ (Difference: $18-7=11$)

The differences between successive terms are consecutive odd prime/numbers: $-3,-5,-7$. Following this pattern, the next difference must be $-9$:

$$\text{Next position}=11-9=2$$

The $2^{\text{nd}}$ letter of the alphabet is B.