Zubat. Nothing is もっと見る annoying than having to go through a cave where there are Zubats and あなた run out of repels.
Other than Zubat, I would have to say the roaming ポケモン like the legendary bird trio. Having to track down the roaming ポケモン is so annoying. あなた can't even trap them with Mean Look since they'll always find a way to escape the battle.
As for the most annoying trainer pokémon, it's definitely Whitney's Miltank.
i'd say psyduck. he waddles around with a big-a** headache, あなた can't do anything about it until it gets to the point where he uses a special 移動する for no reason!