Quoted By:
X/Y or SwSh. They're pretty lenient and will ease you into the concept of nuzlockes while also making you comfortable with the rules, and having a large distribution of Pokemon/areas.
From there, I'd go HGSS -> Emerald -> Platinum -> B2W2 -> USUM in order of increasing difficulty. Kanto games are a little too easy once you've gotten the swing of things.
