Id say nothing, just a lightsaber.. and even then there have been times when a Lightsaber crafted by a sith lord using synth crystals has actually broken through a jedis lightsaber blade.. so not even other lightsabers can stand up to each other.. lightsabers are just epic.
Anything with a cortosis weave can stand up to a lightsaber, but even then it cannot hold up to being held against it for a lengthy time. But due to cortosis' ability to withstand high amounts of heat and energy, in a duel it can withstand the parry's and thrust's of a lightsaber. Read the Darth Bane books for further detail on cortosis
