I think Garen Marek/Starkiller was a bit more powerful the Palpatine/Darth Sidious. In one version of Star Wars The Force Unleashed he defeats Palpatine with some difficulty. He also can Bring down Star Destroyers with his force ability.
I think he is also is a Master of all Jedi and Sith Lightsaber Techniques. He also trained against Holographic versions of Obi Wan Kenobi (ROTS era) and many other famous and powerful Jedi.
He also skated beteen the light and the dark side. In my opinion I think he has more in common with Cade Skywalker who is also enormously powerful and also was neither on the side of Sith or Jedi.
Edit Just as I posted this I saw RealTalents post wow I didn't know Garen did that though