Easiest:
- Flying
- Lentation (Passing through solid objects)

Easy because I've done it so many times, failed a few times to start with but after that it became 2nd nature.

Hardest:
- Remembering Goals
- Energy Blasts (eg. Kamehameha)

Hard because for so long I just followed the dream plot whilst lucid to prolong it and just did whatever I felt like in the moment, and I didn't watch much anime as a kid.