Personally I have done almost every person out there....prefer Jeff Cook and Joe Fargis for jumpers. They are smart, adaptable, and don't talk down to anyone regardless of level of experience. I learned a tremendous amount from both that I continue to use daily. Kevin Babington is also excellent for jumpers.
Mac Cone has a very good clinic system and he's not intimidating to ride for. i found his clinics really interesting and it changed my system of going through my corners and setting up my distance. GM is the obvious choice as well, I found that he went over everything I had already heard a thousand times but all of the sudden it truly meant something. It made sense and I felt the difference.
I've also audited Will Simpson and he's a very easy going 'jumper' clinician. Similar system to Mac Cone.
I recently took a clinic with Linda Allen and it was an amazing experience. She's a phenomenal instructor and teaches you how to teach your horse. Great teacher on cause and effect in riding. Hands down best clinic I've been to.
I've also enjoyed Anne Kursinski's clinic. I audited Greg Best's and wasn't super impressed, but it may have been the horse/rider combinations riding.
Another vote for Jeff Cook. He is one of the best instructors/clinicians I've ever had. He's very thorough and demanding, but calm and friendly at the same time. I really enjoy his lessons, and I feel like I improve rapidly.
"A good horse and a good rider are only so in mutual trust."