Druids!

The Druid class is considered the jack of all trades. Depending on the talent build you choose and the gear setup you can perform a little bit of every role in the game. From CC, Tanking, Off-Tanking, Healing, Melee DPS, Caster DPS, and a wide array of PvP roles, Druids can do them all.

Most of the time the Druid cannot match the raw power of its direct counterparts (cat form - Rogue), (Bear Form - Warrior), (Moonkin Form - Mage), and (Tree Form - Priest), but it has other abilities that these classes don't have that can make the Druid a more optimal choice in certain situations than the primary classes.

Pros

The Druid can transform into a wide variety of shapes and make an accompanying talent build to compliment its shape, making it very versatile and capable of filling almost every role in the game. It has the ability to buff party members and also resurrect allies during combat, an ability that no other class has. By shapeshifting it can also get itself out of snares or polymorph effects.

Cons

As stated above, when you shapeshift into a form it's generally not "as good" as the primary class you are mimicking. Druids are also restricted to wearing only up to leather armor and they are not able to cast in some of their shapeshifted forms.