W

W was the twnety-third letter in the alphabet, which, by most accounts, succeeded V and preceded X, (PROSE: The Dalek Dictionary, TV: Goodbye, Sarah Jane Smith, etc.) however, according to Sarah Jane Smith who recited the alphabet backwards while under mental attack by the Mandragora Helix, she placed U before W. (TV: The Masque of Mandragora)

While unable to directly oppose Ruby White, Mr Smith told Clyde Langer "letter two (B), letter five (E), letter twenty three (W)", which Clyde interpreted as "beware". Mr Smith then added "letter eighteen (R)", with Clyde deducing that he was trying to warn him about Ruby. (TV: Goodbye, Sarah Jane Smith)

An application with a logo featuring the letter "W" was installed on Petronella Osgood's computer by 10 May 2020. (WC: The Zygon Isolation)

Behind the scenes
In the real world, W immediately follows V.