If you had to choose a dark side master who would it be and why?

Ex: I would choose Darth Plagueis because he planned on his apprentice ruling along side him, and would never betray his apprentice. And he would eventually figure out how to cheat death if I didn't kill him.

yeah, plaguies or naga sadow. they were willing as masters, and diligent too. kasim was also a good master. qordis kinda was as well. thats about it tbh. the rest of them were quite shit.

Plagueis, Marr or the Emperor's Wrath.

Kreia

Kreia, Plagueis, or Revan

Now that everyone is saying it, I would also add Kreia

Darth Traya

Jacen Solo.

Darth Traya, yeah. Very philosophically-minded, intellectual, and nuanced, which I appreciate. I also don't think she'd be as physically brutal in her training as most of the other Sith would be.

One who wouldn't physically abuse me.

Aleema Keto. Her "training" of Ulic gotta make her the best master

Sidious 

Honourable mention Traya

Plagueis or Malgus

Naga Sadow

Because being a sith sorcerer is way better than throwing pebbles.

Darth Plagueis for the same reasons as the OP.

Vader because he would whip me into shape and unlike the traitor Galen I would serve my lord (as I do now) until my last breath.

Zannah. She has low standards and isn't so harsh on her apprentices; she let Set get away with a lot of lip, something Bane or Sidious would never have tolerated. She also didn't care that much that Set ran away so if I chafed under her training I could ditch her with the knowledge I acquired without much fear of retaliation.

Vader or either traya

Darth Plagueis, unless the Son was open to taking apprentices and not stabbing them in the back.

Then again, I lack the charisma and sociopathy required to take over the galaxy politically the way Sidious did.

And Sidious already rules the galaxy and told Galen "You could have been my successor, my equal."