Thus far, the instrument was probably the most difficult for me. Just a whole lot of technical information to learn, memorize and be able to process in a split second. Kinesthetically, it really wasn't that difficult, and I suppose that it's because you're not learning how to fly the plane all over again. But you are learning how to operate in a completely new enviroment.
I've heard CFI is the hardest followed by the instrument, but I'm gonna go with Photopilot and say the private was the hardest and possibly also the most rewarding because you go from not knowing anything to flying passengers around.
The toughest checkride should be your private. This is the biggest transition after all, as you go from non-pilot to pilot. A Private Pilot needs to be familiar with just about everything. Every other checkride is just from one level of pilot to the next level.
The type rides were very stressful in preparation but they ended up being "a piece of kelp" (aside: You know you have a 3 yr old when you can quote "Finding Nemo" word for word.)
Private for sure. So much new information, lingo, thought processes-it's whole new world. Tough checkride too. It seems like all the ratings since have just been variations on a theme, yet they all use what I learned for the private rating.
Private was pretty tough, but just coming off the instrument checkride, I'd have to go with instrument. Both are basically two totally new worlds, but for me the instrument had more info that was difficult for me to grasp and learn.
All of the licenses and ratings I earned with Microsoft Flight Simulator were pretty challenging. I sure am glad I have them all done, printed, & tucked in my wallet.
Back in reality: Private was pretty easy, but I worked as a sailing captain for years and there is quite a bit of cross over knowledge that lightened the learning load a bunch. When I consider my current private students the first step does seem like a tough process. The rest of the ratings all had different challenges. I think my initial CFI ride was the most stressful and I probably worked to hardest to get that one.
My private was by far the most difficult for me. There were times during that when I despaired of ever completing FA110, but I got through it ... with an embarrassingly large number of hours for a private pilot, but that's another story. After that, I really had it pretty easy ... my instrument, commercials, CFIs, and even the King Air training was pretty straightforward to me. I think it has a lot to do with what your particular aptitudes are (some people take to IFR easier than VFR flying, etc.) and how well your style and your instructor's mesh. Can't over-emphasize the importance of selecting a good instructor, and not being afraid to make a change if you're not happy!