Definitely the first chapter. When I come up with ideas, they're usually in the middle or end, the climax, which is my favorite part, so I have a horrible time leading up to all the action.
The middle of the story for me, right before (or during) the climax. Usually I know what's going to happen, I just have the hardest time getting there without rushing things.
The hardest chapter for me is the last chapter, because I want everything to tie up nicely. Or, it's the overly emotional scenes because I'm generally not an emotional person, so for me to tap into that and create a scene for it is hard for me to do.
I think that the first and second chapters are the hardest, for me at least. It just seems so difficult for me to begin a story the way that I want to or write down the beginning in my head. As for the second chapter, I want to make it as amazing as the first, which can make it just as difficult.
Everything after the third chapter is usually the most difficult for me, because I feel myself losing control of what I originally wanted and have a hard time pulling it all back together.
My hardest chapter to write is always the first one, because I have a horrible habit at dumping everything on the reader at once, though I'm getting better at not doing that. And don't even get me started on the first chapters of sequels!
The last chapter. For me the beginning is the easiest and then the middle kind of slows but I'm good as long as I have something going on. The ending when the action and everything has finished and I have to figure out how to close out the story. That's the most difficult.
The second chapter, always. The first and last are always the easiest, but the second chapter is always the hardest because you could have an awesome first chapter but a lacking second chapter, and I use the second chapter to decide whether or not I want to stick with the story when I'm reading one so as the writer, I naturally have trouble with the second chapter since the second chapter is always where I find a deal breaker or not in other people's stories.
Lately I've been having trouble with the last chapter. I always have an ending in mind, but it never seems grand or wild enough. I think it's tough to write the ending because you've worked so hard on the story and you just want the ending to be perfect.