I would honestly suggest poison Its not the hardest but its definitely not the easiest. Its really fun with early access to nidoran male (right before Florinia from the game corner) makes the run super fun. I also think that if you want a real challenge I would do the dragon challenge easily being toughest (toughest is an understatement). You rely on treecko if you think about it since mega sceptile is dragon, and on noibat, who is not good in the early game.