Most arcjet thrusters seem to use hydrazine or ammonia, with some research being done on hydrogen and water arcjets.
I know hydrazine is easily catalyzed into exothermic decomposition, and it and its cousins UDMH and MMH are hypergolic with NTO, but those wouldn't seem to be requirements for an arcjet propellant, and hydrazine has the downside of being fairly toxic.
What's the advantage of using hydrazine in arcjets?