i'd imagine none of them because they'd all be bitter to him. at least the ones he'd partake in since there are poisons that can't be metabolized properly by the body, and the ones that can be aren't something he'd want to take more than once. the bitter ones, like hemlock, would have to be taken in small doses so he doesn't absolutely Destroy his liver, and any that taste sweet are usually a one way ticket to an early grave. besides, he doesn't need complete immunity, just enough that he won't pass away immediately, so taste is usually the last thing on his mind.

don't. practice mithridatism irl. chemicals are way more complex in reality than in fiction