Personally, I believe that the Last King is Theseus. First of all, he's was a King. Secondly, he married the Queen of the Amazons--
The problem about him,however, is that his membership in the argonauts was by far inconsistent with some myths.Therefore, there were doubts as to whether he was a member of the argonauts. The other problem is that I am not sure if there were heroes in the myths of other cultures who were similar to him. The last King was supposed to be
Another weakness in this assumption is that I can't see Theseus being connected to the Arthurian Legend--the last King is supposed to have influenced the Arthurian legend--meaning that there should be certain similarities between his myth and the Arthurian one,even if superficially.
Possible candidates would be Jason himself, but he just doesn't seem to be strong at all.
The other would be Heracles, but if I am not mistaken, Verethragna IS Herakles. Besides, Herakles seemed to be too obvious.
But then again, power is myth is not everything. All it matters is the will-power of the god. Hence, it could have been anyone as long as the prerequisites I mentioned are fulfilled.