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Well, finished a Gen 2 only run.
Definitely feels like it could be the hardest, what with both Baby mons and ones that got evos in Gen 4 making up a good bit of the dex. But there's also Steelix and Crobat. Steelix was definitely an MVP with Stealth Rock and Ice Fang, Crobat for any fighting or grass types.
Tyranitar was obviously good too, and Meganium put in some work. Smeargle just used Salt Cure, and Donphan was mostly just there.