I'll go with Gold/Silver/Crystal or HeartGold/SoulSilver, but I'll also agree that "challenge" isn't really a thing Pokemon does without some outside guidance. The biggest reason why I'd say that is the spike in level increase after clearing Johto, as well as the Red fight being one of the highest-levelled, not-scaling fights I can think of (albeit a basic lineup). There's also stories about Whitney's Miltank, though I can't remember her giving me trouble way back when.

Definitely use Set mode, and probably consider not using items during trainer battles or limiting yourself to two items during trainer battles.

Quote Originally Posted by tyckspoon View Post
It's also easier because 'smart' AI wasn't invented yet - Gen 1 opponents just randomly select one of their available moves, so you can gamble and get away with setting up on things that should kill you because they don't know they can kill you and just try to use Defense Curl or something three times in a row instead before you smack them with a Psychic.
I think it's random selection for wild pokemon, but I remember Twitch Plays Pokemon somehow ended up exploiting Gen 1's AI. They had a level 36 Venomoth take out Lance's Dragonite with mostly poison damage and Gen 1 Leech Life, because Dragonite only used Psychic-type attacks (Agility and Barrier) against a poison/bug type.