Definitely Rock Tunnel, because it requires the use of the most useless HM's in the game. You need to teach a pokemon cut to get to the cave and flash right after it in order to navigate through the cave. Of course you can also get through without flash, but it is way more comfortable with flash. Also I usually have to get back to the Pokemon Center halfway through because my pokemon took a lot of damage in battles or were poisoned.
Victory Road can be tough aswell. It's annoying to encounter Zubats/Geodudes every two steps; and you need strength. At least there are trainers inside the cave.
Victory Road can be tough aswell. It's annoying to encounter Zubats/Geodudes every two steps; and you need strength. At least there are trainers inside the cave.