Best and Most valuable class to play as a Warlock?

I suggest if you want to play a really OP class, then try out a mage. My friend was hitting around 600 with pyroblast on his lvl 30 non-twink fire mage. some of the pro's and con's are:

Pro's:

Tons of sustained damage at the Endgame Raiding level Hyjal/BT and beyond

Very little mana issues can be completely self sufficient with the use of Gems/Pots/Evoc at correct times.

A decent raid debuff in scorch for a variety of people in the raid.

Large range for single target nukes.

Tons of Crit based Fire Talents and Mana Regen on crit

Plenty more pros but this is really all thats worth noteing (Speaking in the lines of boss fights)

Con's:

Very low survivability

At the cost of consumables such as Destro pots / Flame Caps unless your grouped with a SP and or Resto Shaman you wont be able to completely maximize your full DPS potential

High Threat but also very controllable

Not really the best spec for fight under 2-3 mins in length due to the sustained damage viability rather than that of massive burst. And possibly one of the weaker mage specs for clearing single target trash due to lengthy cast time of main nukes.

This is just a couple of things ive noticed speccing between the 3 trees. But i will say that if you can get in a great raid with the correct composition you will have a helluva lotta fun with a deep fire PVE build.