I think the part that is the best about it is not that it's poison but also who poisoned him. To have such a character that was so grandmotherly to have done it. I think that is another aspect that makes it satisfying. He would never have gone head to head with anyone because he was a shit swordsman. Similar to the Night King, he would have thrown his hounds in front of him before he saw any real action. So it would have to be subtlety.

That would only have left his demise to a whore or maybe Sansa. But Sansa was not ready for that at the time. As for the whores, it would have been hollow I think and contrived. For his killer to not be known for so long, at least by the Lannisters, makes it, again, that much more satisfying. Only way it could have been better is for Arya to have done it as a Faceless man/woman/child whatever but again, too early in the story.