Either Ziost or Dromund Kaas, but it kinda depends on what time period for ziost so most likely Dromund Kaas.

Dromund Kaas, specifically Kaas City. Korriban is my second choice as it is the home world of the ancient Massassi sith species

Throw em out into Korriban. If they survive, they might be worthy of a Lord’s attention

Korriban sounds tempting due to th history of the Sith there and the Valley of the Dark Lords. But I'm going to go with Malachor V as the entire planet is a nexus in the dark side. Revan brought Jedi to the Trayus Academy and through sheer force of corruption on the planet turned them to the Dark Side.

Didn't expect Malachor V to be the second

Exegol

Korriban. Challenge me!