What is the best class in World of Warcraft

There is no overall "best" class in World of Warcraft. Each class brings something unique to the battlefield. One class might be good at AoE grinding while another excells at single mob killing while yet another is better at healing.

Even in the three different main roles (tank, dps, healer) there is not really any one class that far exceeds other classes in the role (ie "the best").

What it comes down to is how well the individual player knows its class and can perform its functions. A very well geared player may not always know how to play his/her class very well because they were dragged through content (ie instances or raids) by other people. At the same time you might see some undergeared people do really well because they know what they are doing (ie know their class spells and what the proper time to use them is).

However each class has advantages over other classes, however one must know how to play that class, and the talent tree they are.