You can look at it from multiple angles.
You can look at the number of skills you have to manage and use that as a basis. In that case Engineers and Elementalists seem “harder” because they have more skills to manage. But you can also argue that since they are all limited by cooldowns, all you’re rally doing is rotations.
You can argue that it’s about risk/reward. Playing a glass cannon well requires a lot of finesse and situational awareness since one mistake can undo you. In that case Thieves would seem most challenging since they lack any form of passive defense. But again you could argue that all they do is “spam 3” or “spam 2” because they don’t have cooldowns.
You could argue that playing a Bunker requires most skill since you’re the most important member of a team and have to keep track of both your enemies and your allies and assist accordingly. Your success probably has the biggest sway on the outcome of a match.
You could argue Mesmers and Engineers take most skill because they are the least popular and most non-intuitive classes. And that must obviously have to do with their higher skill requirements.
It’s all about how you want to twist your arguments.
Imo though playing any class “well” isn’t difficult in any case. Sure, grasping a Warrior might take slightly less time and practice than grapsing an Engineer or Elementalist, but both can be created and played at a competitive level within a day.
What really dictates your success in PvP is situational awareness and coordination with your team.
