Mara Sov no contest if you consider her to be light because she is half. Otherwise, Kabr and I'll explain why: -Kabr tore a wound in the Vault of Glass and let in the light, a place where the Vex should be near omnipotent. -Kabr created a artifact using his light that literally let us say 'NO' to the Vex whenever they tried to delete us. -The relic It forced the Vex to accept us as a fact of existence in their timestream and Kabr's will was so great that it couldn't be expunged. -Kabt basically tore through reality to crack the vault. No other Guardian did anything nearly as impressive as what Kabr did. And mind you, he had no prep going into the Vault.