Azurill in my case. I could barely run into any at the Ranch and the very few that I did were nigh impossible to catch. Ugh.
Originally Posted by Master Rayquaza
The monkeys were hard. But, without taking the shaking grass into account, the hardest was probably Riolu.
Oddly enough, I ran into a Riolu straightaway on my B2 game and caught it with no problems. It even had a pretty decent Nature: Naughty. Guess I got kinda lucky in that regard.