Agreed, Geoff Lawton's is the highest quality online PDC. Below that, I would recommend
Permaculture Visions because it is one of the oldest and longest running online PDCs; it costs less, and it can be started any time of the year. Another one to look at is
Open Permaculture which is more for the visual learner, the other one is reading based, and it costs less than both of them. At the moment, I am taking
Permaculture Education Center's PDC, and it follows a reading based course with homework and responses from instructors. So far, I like the course, and even though I've read Gaia's Garden by Toby Hemenway, there are still some things to be learned. Then, for a biggg listing of all the options out there,
Permaculture Paths lists many things ranging from online to in-person.