So far, it has to be Smoochum. Yes, Smoochum. But this is the only one I can think of where I had a few issues, and I haven't gone into the wild to catch a lot of Pokemon. XD;
Well, there was a swarm one day, so I went up to the lake to go catch them. I have a False Swipe Pokemon, so I used that. Then, I figured "hey, it's a Baby Pokemon, can't be THAT hard to catch" and threw a Luxury ball at it (I couldn't remember if it evolved by happiness or level, but it couldn't hurt either or). It broke the first time, which shocked me. .-. I threw a few more and I think I got it with a Ultra Ball.
Oh wait! Croagunk and ...I think it's Drapion... are extremely hard in the Safari Zone/Marsh Game. Not only are they rare, but they're always a pain for me to catch. I don't even think I caught Drapion they day I saw it.
I hate the Safari Zone sometimes. I'd rather pay the extra money to go in the wild, buy pokeballs, and have my Pokemon weaken them. .-.