It depends on how you play, there are pvp builds for all 3 of them as well as raiding builds for all 3 of them. If you have a spec that you like you should go to a website that shows you the best rotation for that spec for raiding or pvp and which stats you should have and what stats you shouldn't have.
It depends what you want to do. For PvP, I find a destro/demonology build best, but if raiding an affliction/demonology build is brilliant.
Depending on what the build is used for (raid trash, bosses, solo play), an Affliction build is probably the best all around. A Destruction build might be better for clearing trash in instances, but will use far more mana than Affliction. With Affliction, you also have the health returns from Soul Siphon and other talents.
i know that the best build for leveling a warlock is definitely Demonology but not to sure on rotation :/
Like most questions in this category, there is no way to identify a single talent that makes or breaks a warlock. Simply play around with the talents, or ask an experienced warlock player advice on what talents to take should help you much more.
Demo for raid utility during 25 man raids for the sp buff. Destro for large hits and PvP Aff is currently the highest dps build, and used for long boss fights and PvP as well.
my best friend has affliction
Envenom with the build that is in the related links
Affliction is a noun that means pain and suffering. Affliction is often used to describe a disease or illness.
Undead.
In the related links section below are two builds, one for tanking and one for dps'ing.
Legendary Warlock's staff
obviously the marauder because it is the longest, has the best quality foam, and fits better in the tacticool vest. now if the people you are playing against are wimps, then the warlock.