Hariyama and Lairon.
Hariyama was bulky and hard to OHKO for some Pokemon (not counting super effective), so most of the time it took multiple hits to KO it. What's bad there? Whirlwind. You use up few of your Hydro Pumps on it, and when it reaches the critical area, it uses Whirlwind. You encounter it, attack it once, it uses Whirlwind. You attack it and miss it, it uses Whirlwind. Whirlwind Whirlwind Whirlwind. Worst place for grinding.
Lairon too. Worse than just Roar is having Protect too. Hydro Pump can easily OHKO it. What does it do? It uses Protect and you waste one Hydro Pump, and in the next turn it uses Roar. Perfectly wasted Hydro Pump.
Of course, it's not a big deal during normal plays, but I barely finished my Sapphire Nuzlocke because of them. It took ages to train everyone. If it werent for the speed button, I'd probably quit or just overlevel one Pokemon and try to solo E4, which would end up bad due to my terrible team.