Though none of the mainline games are that hard, which one of

platinum

>>27365406
Probably Ruby/Saphire, or Platinum.

Ruby/Saphirre was tough to find a competant pokemon to use, and Platinum buffed the champ and E4, while Giratina was rather poor in-game.

The first half of GSC were rough, too.

>>27365406
BW2 Challenge Mode

Yellow

Gym 5 has level 50 pokemon

>>27365413
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RBY were hard. After beating everybody, one of the few ways to get money without cheating was Pay Day or taking on the Elite Four. If you wanted to buy something, you had to farm the Elite Four. TMs weren't reusable and you had to buy more is what I'm getting at.

>>27365427
>BW2 Challenge Mode

Probably the closest.

The early games were only hard when you were a kid and didn't understand the types and mechanics completely.