It's gotta be Rattata. Every time I was in the grass, I ended encountering them in almost every step. I've had to spend tons of money on Repels just to get away from them, haha. Due to the same reason, Zubat and Tentacool were quite annoying as well. Plus, it was quite hard to defeat them if your Pokémon were under level 10, because they had a tendency to confuse you a lot with Supersonic. D: