I don't think there is a thing such as the best series. Different coaches have different train of thoughts and you got to figure out who you can wrap your head around best for yourself.
Fwiw for me it's the "adjust, adjust, adjust" mantra that I find to be best illustrated by Blah, Grindcore and FoxwoodsFiend, along with a number of other coaches.
It depends on what your skill level is and what areas you are trying to work on, but these really helped me for microstakes (there are so many more good ones though, including stuff from sthief, threads, and other of DC's great coaches):
Coaching Kristy 1 - Although it's a small stakes video, covers all the basics (and more) of how to crush microstakes 6max
Handreaders - Covers the fundamentals of handreading
Full Ring Evolution - Covers all aspects of today's (2012) full-ring games moving from 10nl through 50nl
The Thin Red Line
The Thin Red Grind
Coaching Kristy 2
Of course, you can't just watch video series, and expect to crush overnight. You're going to have to watch vids, read and discuss hands on forums, go over your hand histories, and figure out where you are leaking money, constantly look to correct your mistakes, and constantly look to make the most +EV play.