There is no contest here.
Rogues.
Granted there are other pain in the arse classes, like warlocks, but they have much more weaknesses, and have fewer escape options. I PvP with an enhancement shaman and a holy paladin. Both those classes have no defense/survivability against rogues.
With mace stun, they can basically auto attack you to death and not let you get even a single attack off, and allows them to spend their combo points on more offensive attacks. There has been COUNTLESS times where I was taken from 100% to 0% and not even able to get a single attack off, except when certain cooldowns were up (i.e. blink, divine shield, ect) but all those do is just prolong the an already inevitable losing fight. the biggest exploit they have: THEY COMPLETELY NEGATE ALL CASTING ABILITIES, which makes some classes completely helpless against them. With the dozen different stuns, and of course, kick, which silences a player for what feels like half an hour. They make it so healers cannot heal, and they have anti-healing poisons that would make mortal strike warriors blush....
They can stealth, freely pick their fights, and ambush you from out of nowhere with no time to prepare, keep you stunned the entire fight and simply prevent you from getting away, should you somehow break free, with deadly throw, slowing poisons and sprint.
If in the rare occasion the fight actually DOES turns against a rogue, they have more escape options then every other class COMBINED. Evasion basically gives them complete damage immunity for a good period of time, cheat death basically adds 10k HP to their heath (a pyroblast critting them for 20 damage!? C'mon...), and can tank multiple players even better then a prot warrior with shield wall on. They can vanish and disappear in a split second and completely remove themselves from combat even when there was several players actively beating on them. They can stun and sprint away, they can blind a player for 10 seconds and run away. They have more collective CC then any other class. I once saw a rogue sap 1 player, blind a second and cheap shot a third, effectively single-handedly removing 3 players from a fight. No one else can do that.
With all that they are the hardest class to kill, have the most CC, and the hardest class to survive against. They are the only class that has never seen a significant nerf, and has actually been buffed every patch, even though its the class that least needs it. And since all Blizzard development employees play rogues, we are not likely to ever see them nerfed. Ever.
-VGsatomi