It's tough to say, because some games have a huge gap between using the Pokemon they throw at you and the ones you want to use. Johto games are pretty hard if you're using shitmons, same with BW. But if you're using Typhlosion/Amp/Espeon/Water Type or Starter/Monkey/Stout they're piss easy
For someone on a first playthrough BW2 are probably the answer for hardest, especially if you consider World Tour to be during the main campaign.
Firered/Leafgreen are really easy. There's not really any rival fights or gyms that are close to other games.
>>43604299Gen 6 and 7 essentially have a hard mode in turning off the exp share. USUM without exp share is almost at challenge mode.
