That's like asking which is the best tool for a toolbox, or the best brush for painting with. They're all useful, you use them according to the situation you're in. Watch the Drift Bible for a good introduction to all the various ways to initiate a drift and examples of scenarios where you would use them.
The best place to start, though, is with the basic power slide. That's where you go into the corner normally, like a grip driver would, but once you've entered the corner you give it a bit more throttle than a grip driver would to break the rear tires loose and the rear-end should slide out naturally. It's easy to initiate, so you can just focus on keeping the car sideways while managing speed & angle.