Zubats have driven me crazy ever since the first time I have laid my eyes on it. No repel in a cave this is what happens - In five minutes I see about 400 Zubats coming along. Some use Mean Look and nasty things like that, so I end up fighting them and confusion.
Next is any Pokemon with Arena Trap, etc. Trapinch. It's annoying when your desperately trying to head to your destination without a good supply of repels and Pokemon with Arena Trap pop up. You can't flee and your forced to fight them.
Last is Roggenrola, it's ability Sturdy, drives me crazy. It's like a waster of PP, since you can't kill it in one hit.