What do you consider an interesting Reverse CTF challenge ?

I’m going to create my first reverse challenge for a school project (Already created some others in Pwn, steg)

Before starting to design it, I’m trying to gather some opinions about CTF players.

What do you personally think makes a good reverse CTF ? What would make you go « This one was fun to solve » compared to a boring one ?

Is it about difficulty ? Is it about the stuff you need to decipher ?

Curious about the opinions of both beginners or experienced players.