Strongest Apprentices, Weakest Masters?
Talking about Sith Masters/Apprentices here, that is, in the "Darth Vader"/second highest Sith around sense of Apprentice, not padawans.
Of the various Sith apprentices who never claimed the top spot, who do you think were best, strongest, and most deserved to be Masters?
Of the Masters who had the spot, who should've never progressed beyond apprentice or otherwise weaker?
For times outside of rule-of-two era, count whoever's atop the Sith in question (be it the Sith Emperor, Kaan, Krayt, etc.) as a Master, and only the top other Sith as Apprentices (those on the Brotherhood's council, the One Sith with specific titles like Hand or Voice, etc.). And anyone else, is too low to matter.