I'm a bit sorry for the obvious point, but... Kaladin? I mean, we don't really know what Szeth can do with Nightblood, but he's already lost once...
Also, the following are assuming his previous skillset (Honorblade):
- Susebron: I imagine the ability to Awaken all the things would just be too much for Szeth
- A Decent Earthbender: Assuming that an Earthbender has the knowledge of how Stormlight is stored, they could just separate Szeth from his gems and use their then-superior mobility to batter him until he dies
- Shadowcat (Kitty Pryde): I notice that the focus has been on someone who could kill Szeth, but if someone can consistently escape from him, he can't kill them.
- Steel Compounder: Same sense as the previous assertion.