There are several VERY different curricula that are all called JKD, that are legitimate based on lineage from Bruce Lee.
The main two branches are
- Original JKD aka Jun Fan JKD or "The Nucleus" (under Ted Wong, Taky Kimura, James Lee), and
- JKD Concepts (under Dan Inosanto, and including Bustillo, Hartsell, Vunak, etc. Even teachers like Demitrius Barbito, Kelly Worden, and Matt Thornton are from this lineage--and their material all looks VERY different).
The "best" depends on the criteria by which you're judging. Are you looking for the program most similar to what Bruce Lee himself did? Or the program that applies his ideas the best, to teach you to fight well, the fastest? Should it include defenses against knives? Should it include grappling? Should it teach the exact kind of grappling the Bruce Lee did, or should it include the modern advances that most JKD practitioners use today?
Figure out what you want, and look for the names I've mentioned. I personallly am a fan of Matt Thornton and his associate Luis Gutierrez, but there has been tremendous evolution from what Bruce Lee himself practiced, to the point where they are really doing more sport MMA than anything else. Their teaching is very systematic and logical, and easy to follow... but the content may not be what you want. For applications of JKD to street fighting, Vunak and Barbito are very good. For the really classical Bruce Lee curriculum, look for videos with the names I mentioned under "original"/"nucleus".
Videos are much better than books when it comes to learning physical skills.