Spy cause it requires, in my experience, good psychology. Just like 4ks said, you need to predict the enemy's movement and next actions, know when to strike and who to strike. Cause distraction for your teammate (I.G Taking down buildings and vunerable classes, spam dead ringer, etc).
Survivability is top priority (Obviously for all classes duuuuh) cause one rocket or pipe and its over.
In order to use the purpose of the spy, he must risk his life getting closer to the enemy to backstab which can be situational.
So yea, spy is kinda hard.