Have taken tons of classes, mostly by going to Quilting by the Lake in Upstate New York for many years starting in the mid 80s. It's gotten very expensive, but if you can afford it - an experience worth having.
Also, at my LQS and when one of my Guilds brings in a top teacher. My first class was with Karen Berkenfeld at FIT in NYC in the 70s; still use the scissors she gave everyone.
The best classes have been with: Nancy Crow - fabulous, really stretches you; Maurine Noble from whom I learned machine quilting and thread uses; Jean Biddick - the best piecer I've seen and from whom I learned about IBC silk super-fine pins that sew over easily w/no damage to the needle; Margaret Miller just cause I love her.
Those would be my top 4. There were many more from whom I learned alot, made some friends from around the world, and had a good time.