What's the most challenging Pokemon game for single-player?

[Taevyr]

Nuzlocke and similar challenges are the only way to make them really hard.

Beyond that, the only one I remember struggling with (having played all gens up to and including black) was Platinum. And I'd probably breeze through it in adulthood.

If you're interested in trying unofficial pokémon games, I'd recommend pokémon reborn: it's a bit edgy (certainly nothing too bad, just want to say it), but quite challenging, especially in how limited the initially available pokémon are. You're essentially supposed to have a large roster to switch in-and-out depending on the gym you're facing. Got gyms for all types, all pokémon up to sun & moon, and plenty of content though it's as-of-yet unfinished.