Modern sensibilities would agree with Saber holding the correct argument and being in the right, and her the most moral.
Regarding the argument itself, philosophically Iskandar absolutely manhandled Saber, tough you wouldn't be far off if you called his arguments sophistry instead. Regardless, Iskandar was able to easily bully Saber into being speechless because her idea of "martyrdom good" was super easy to tear apart by appealing to basic humanity, and to the agency of the king's subjects. He was able to paint the picture of of Sabers kingdom as one where its subjects were babied by some self-destructive ruler who seemingly abandons human nature for the sake of the collective, and while many can appreciate the spirit of selflessness and self-sacrifice, it can also he alienating as hell to the degree Saber took it. At that point it no longer mattered that Iskandar's own empire collapsed after his death due to him caring more for conquest than governing, Saber had already cornered herself, and made it look like her subjects willing choices in life were wrong and second in importance to her perception of ideal leadership. Iskandar embraced his own failures while making his army look like a merry band of brothers, and at that point there was nothing one could say to him that he couldn't deflect with a "yeah I know and that's fine, but ur shit."
Regardless of whether Iskandar is in the right or not, he trolled Saber hard. Outskilled her in speechcraft and din't let her get the reins on the conversation again, even though in theory she had the more solid arguments. Rider turned the conversation to be about the kings rather than the kingdom, and used that to Squash Saber. His Noble Phantasm was the finishing blow.
Meanwhile Gil just sat there and enjoyed the beatdown, safe in the knowledge that he had the best results to support his claims of "I am indeed the best, but that was well established already".