A lot depends on the learner and the commitment. Learning the Thai letters is important but often skipped as I did when I started. This is a very basic class that can also be learned individually prior to taking a class.
Started with YMCA. Learned some Thai. Met some great people. More of a casual learning environment.
AUA is much more serious and intense. A little intimidating for the novice.
Payap is even more intense. Assessment prior to enrollment. As stated by another this is more geared for language fluency. Good teacher and class.
Never tried CMU.
Not all classes are geared for all learning styles. Sit in on a class before signing up.
A lot can be learned by a dedicated learner without classes. Many great tools available now.