There is no "best" per`se. It all depends on what you want to accomplish.
If you enjoy decent dps, the ability to solo (usually even level elites), not having to always be looking for a group, being middle of the road in BG and raid dps, and running around with a killing machine. Choose Demo.
If you enjoy dotting up a bunch of mobs, being a tad higher on the dps chart for raids and bg's, and having little to no downtime when grinding experience. Choose Aff.
If you want to be an all-out dps lock, and you're not concerned about being out of mana semi-frequently...I'd suggest going with Destro.
Each tree has it's strong points, and it's weak points. WoW Wiki is a good reference point for talent trees. There is no "best" tree however. It's going to depend on what you want to accomplish, and preference. Good luck.
Siafu