The strongest class currently is the nuker. The latest patches changed the attack rating vs higher level players so the nukers will miss more attacks on any higher level player.
If the fight is a same level and each person is high enough level to have a decent build for their class the nuker will win hands down. Although nukers take a lot more effort to build for PvP they are the strongest. If you have the 500 defense clothes and a 200 defense trinket, maxed your defense buff and have a strength relics you will get 2k defense to tank warriors no problem.
The strongest for PvE could be any class like everyone stated here, warrior would be the least chosen because they also need gear to be very good at PvE. If they have damage absorbed into life relic and a vital energy absorbed hat they can last a very long time using only their active skill until they hit the ice map.
A healer hybrid for PvE are pretty simple and you do not need any gear really besides maybe vital energy equipment for the hybrid to last longer before meditating for vital energy. A healer may want some HP clothes since their penalty stat is in constitution and at a lower level where their buffs are not max they may want to get as much hp as they can or they could be healing themselves every mob kill.
The nuker for PvE is slow at first until a higher level when they've got their nukes to a decent or max level they can take down mobs in two hits, unless they are in the ice map.
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