This is a super tough question! I think in terms of where my understanding was at the time I did the experiment, one of the hardest experiments that I did was for my A-Level physics coursework. I came up with my own experiment that used ultrasound (high frequency sound waves) to measure the position of a swinging pendulum on a string. I then made graphs of its motion and basically derived the maths of decaying simple harmonic motion – although I didn’t realise that this was what I was doing at the time! Simple harmonic motion is found across all areas of physics so it was pretty cool to rediscover it in my school classroom!
I’ve done loads of really interesting experiments over my time at university, including lots of nuclear experiments with radioactive material, but I don’t think I found these as difficult as there were always experts around to help me out when I got stuck. I was completely on my own doing my coursework back in school! I’m sure I will have lots more difficult experiments to get stuck into in the future 😉