In my opinion, Koffing is the most under-rated mon in the entire little cup. For me, it's niche is being to wall both Fighting Types and Drilbur. There's not a lot of things that can do that well. It has just the right moveset with Clear Smog to beat Timburr and quite a lot of Scraggy's. Will-o-wisp just makes loads of physical attackers useless like Mienfoo and Drilbur. Pain Split isn't great but it's better recovery than Bronzor gets. As oiawesome pointed out, it can wall all the other random physical attackers like Pawniard. I know they're much more rare but it's a useful niche.
It's not A or S, but I don't think it's C either. B seems like a solid position for Koffing.
(Also, Koffing+WishLickitung is a great defensive pair, go use it.)
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