what is the next letter in the sequence WITNLITF ?

S it’s the first letter of each word in the question.

Here's more puzzles that can get your brain cells working:

K C M S F ?

A B D O P ?

M H C K R ?

M M B M M A M M M ?

Find more at Master Of Logics.

Check out some Tips for Brainteaser Questions shared by Optimize Guide.