The answer to this question changes wildly depending on what aspect of the game you're talking about. General world PvE? Event bosses? Dungeons? Structured PvP, and in which format? WvW zerg? Smaller WvW groups?
sPvP tends to get the most attention from people concerned about balance so you'll see people say warriors a whole lot, as they're pretty strong in that regard.
The range/melee thing isn't class dependent so much, as all the classes have some sort of ranged option. It's true that range is much more advantageous in PvE though. PvP wise it's Warriors that have been called OP the most. That's probably just them having a particularly good burst strategy in a meta-game where most players are going full-on glass cannon. Still, they're definitely a potent class; very good mobility to go along with the damage.
the game is more base on "player skill" than the stats and build of the classes.
If you're good at dodging (especially as a thief) every class can be a powerhouse. All of them have some sort of support skills and DPS skills and you can switch between them easily in a middle of a fight.
so has of right now I don't think there's is an OP class, some might have it easier but not enough to call them OP.