I nominate Darth Nihilus to be the #2 Sith Lord. Allow me to explain:
“One cannot have power of that magnitude and still think and perceive the universe as we do."
"The extent of his power cannot be put into words… and his perceptions have grown as well. To him…"
"You are dust motes in a storm… a grain upon the beach… and as insignificant as a body that orbits the graveyard of Malachor."
-These accolades alone should show that Nihilus is, well, an incredibly powerful sith lord, easily among the greatest in history. Furthermore:
"Nihilus powers grew so extreme that, for most beings, mere exposure to the Sith Lord would cause immediate and utterly mindless devotion."
There are only a few sith lords capable of this (including Darth Sidious and Emperor Vitiate), i.e. having such great power that their mere presence can weaken others. Now, on to the feats:
-Ripped the Ravager, along with his entire fleet, from the immense gravity vortex of Malachor V. This is arguably the greatest telekinetic showing ever.
- Used telekinesis to overwhelm one as powerful as Darth Traya, then proceeded to drain her force energy in but a few seconds.
- Drained the energies of entire planets to fuel his sustenance.
He was only truly defeated because not only was he starved (hence why he came to Telos to feed,) but his attempt to drain the Exile further weakened him. In my opinion, in terms of pure power, he is very much comparable to the Sith Emperor himself.