There is no "best" talent spec. They are all equally viable, though they have totally different playstyles. How do you like to play your lock?
DoTing and Draining with practically unlimited mana? Affliction.
Have a powerful pet that you support through less powerful DoTs and Drains while his damage and ability to hold hate make up the difference? Demonology.
Nuking like a mage but with a few more tricks up your sleeve? Destruction.
They are all powerful forces to be reckoned with, and all have thier strengths and weaknesses, but are all equal.