As for what is the hardest plane to fly, I can't point to anything specific other than the one you are afraid of. After you get a few planes under your belt and you seek proper advice on set up and flight characteristics there isn't much to flying any plane except to relax, take a breath, and think about how the plane should respond to your inputs, what it actually is doing, and why is there any variation in the two. Most of all remember that this is supposed to be fun. After all, it's just a toy

You can read literally for weeks on end in the aerodynamics forum about what makes a plane stable and what makes a plane behave badly. Look for a high wing loading, small empenage, short tail coupling, and low power to weight ratios to indicate a more difficult aircraft. There are, of course, exceptions to these criteria as well as other factors to consider, but these are good starting points.