It's hard to say. He's an extremely potent telepath; enough so that Luke (at least as of early NJO) couldn't resist him on his own. He specifically can create false emotions, implant false truths, and trigger false pain in his victims. He's easily amongst the best at what he does, if not the best.
Telepathy is by far his strongest suit, though. He's beastly in other areas, but that's his primary strength and what made him so dangerous. (Excluding what happened after he accessed Coruscant's Force Wellspring of course.)
Originally posted by NewGuy01
I think it's only worth noting that he was good enough with it to pose a threat to Luke through raw power, as he possesses little skill or finesse in it's use.
It should be noted the he was a threat to Luke, Mara and Tahiri all at once, not just Luke. Those 3 were unable to overcome Nyax in lightsaber combat 3v1.