Well first of all I'm not sure about what you mean by "the best edition". Second, I have not used formally Jackson for any of my EM courses (I'm still undergrad) but I've been through some parts of the book using its second and first edition. Many of the information that I will tell you is on the Introduction part of the 3rd edition.
You see, the main difference is that the 1st and 2nd have this chapter on Magnetohidrodynamics, which the 3rd edition doesn't contain. Then there's the issue with the units, the 3rd edition uses SI units for a large part of the book (unlike the earlier ones) but returns to Gaussian units in the last chapters (you may or may not find this a good thing, depending on which units you are more comfortable with, I think).
I don't remember everything that the Introductions mentioned but you can find them all three in the 3rd edition and then judge by yourself.