Don't know how many people have had formal art instruction, but there must be an art instructor that you wanted to take a class with or had the pleasure of taking a class with.
I'll get the ball rolling with the mention of my instructor
John Clapp. He teaches Intro to Illustration at San Jose State University and I highly recommend him. After taking a class with him, I looked back on the prereq classes and felt like I could have circumvented it all by just taking his class.
He does an exercise every semester where he has students do a charcoal pencil self portrait the first week of class, and then a full blown reverse drawing the last couple weeks of class. The difference in 15 weeks is amazing. He posts up the before and after drawings in the display case the following semester and commonly hear people walking by commenting, "That drawing is pretty bad, but that other one's pretty good!" They just happen to be referring to the same person's before and after drawings.
He gave us a few mantras to remember as we continued through our education:
"Contrast is the meaning of life."
"Shadows are your friend."
"The answer to everything is squint."
He taught us what to look for in our drawings and ways to think about them. "Shapes, tones, edges."
"Correct it when you see it." This came in the middle of a foreshortening exercise when he told me to measure two distances. After which, I erased half of my drawing.
"You'll always just suck less." This was in reference to the fact that most artists can always find something to improve on any drawing they're working on and their own abilities and that you're your own harshest critic. It helps me keep a little humility when my head gets too big. I liked the saying so much, I found a sticker for him that said, "Sucks less!"
He also gave me the best piece of advice, which I have yet to truly take to heart, "If you drew as much as you played games, you'd be the best artist in the world." Alas, I love my gaming a little too much to give it up.
But being on this forum keeps me from getting rusty.
My friends and I have so much deference for him that we call him "The Great JC" behind his back.
He has an Intermediate Illustration class that teaches color theory and perspective, but I never got a chance to take that class. Doh!
He does children's books in his down time and I'm proud to say that I've been to a book signing for each one of his books..
Anyhow, that's my favorite art instructor. Next?