Papatine and Vader had many trainees, one of which might presumably have gotten a promotion in the event of a death somewhere. There were dark side prophets (who specialized in listening to the future), the Emperor’s Hand (of which there may have been several), minor dark Jedi and servitors, and odd experiments (such as Azrakel and Joruus C’Boath).
None of them came close to matching either Palpy or Vader, however, though a handful were arrogant or stupid enough to think so.