About Nihilus, he was actually really strong, even without his force. While his draining ability was in a way a weakness and kind of a curse as he had to constantly drain the force to keep himself alive (draining an entire planet only kept him fully energized for a short while), he was able to drain all of the inhabitants of an entire planet which speaks volumes, however we do know that he needs preparation time to pull it off in one on one combat, as he had to stun the Exile and his party first before trying to drain him.
He was also able to escape death by containing his consciousness within his armor, pretty impressive which most likely required some pretty good force mastery as well as knowledge.
His will was so strong that he was able to hold his entire ship (which had been shattered into pieces) together through the force, which he actively did, constantly - pretty impressive display of TK imo. He was also able to dominate the minds of the crew of his ship, and install his sith assassins with a downgraded version of his force drain.
Also, while he was defeated by The Exile, Visas and Mandalore, this was partly due to the fact that attempting to drain the Exile weakened him like hell, and Visas and The Exile especially were still pretty strong anyway, while Mandalore was also no joke either. And the fact that he was able to instantly stun all three still speaks volumes, if he hadn't gone for the drain, he would have most definitely defeated them.
He was also able to do this to Sion - http://youtube.com/watch?v=IobIhctxRXg - ownage. He was also able to cut off Traya's connection to the force, and this was seemingly before his rise in power - http://youtube.com/watch?v=ENj8jTQEbvU . Damn impressive imo. I'd say he would be a good league above Malak, Sion, the Exile, Traya, and probably only just below Revan.