I do mostly PvE and the class/spec that ive noticed to be rare would be feral druid, I've seen two good boomkins but never feral. I honestly haven't used druids or have i played a few others so dont know if it is the hardest but thatd b my opinion.

How do you feel on mages and priests?

We'll see in WoD, but Rogues were hardest for me. It felt weird that a pure dps class couldn't out dmg my fury Warrior.
I felt like I was doing something wrong lol. It could also be that I've been playing Warrior since Vanilla though.

I heard Ele Shams are fun?

or priests

Looking for more!

Feral is incredibly fucking hard.

I'm pretty much stuck as bear tank 24/7 'cause I cannot learn feral.

feral is decently doable once you realize its a priority system and you get down every case that could happen with a priority for each. Granting, GETTING to that point is kinda hard, but I have always been feral dps and ret dps as my mains since BC, and have always pretty much only been able to play feral and ret due to the complete lack of brainpower needed for any other dps class, they all bore me to death.

rogue

I'll awnser for my part in the PvP since i don't PVE much. From experience in PvP, the hardest i have played and the one it took the longest to learn was the feral drood. I am NOT saying it is unplayable, it IS playable, but even when you learned it well, timing your abilities and cooldowns for the max DPS on people is still hard. Managing your instant heals so you don't waste them, using deff cooldowns to stay alive and survive bursts, that is that i think is hard, you can do it, but it takes a lot of time to master it well, and when you do you can 1v1 any class, simply because the feral has one of the biggest survability. I did carry A LOT of people in Pandaria, in 2's and 3's, and i don't think i could have went that far without a Feral druid

ALSO, i think mastering Shadow Priest in PvP is also * Hard * Because it has a lack of survivability. Yes it is good and has a decent damage, but mastering how to stay alive, kite well and use your dps in an efficient way is hard IMO