There is often talk on here about what constitutes a good public class - of people complaining about masterclasses and praising them.
I'd like to share this example on youtube of what I think is really
representative of piano teaching at the highest level. It's a 1987 masterclass with Georgy Sebok - one of the greatest musicals minds and spirits of the last century. For me, what puts his teaching in a class of its own is his eloquence and concise articulation of WHY he thinks the music needs to go the way he suggests.
I won't spoil it for you, but he gives detailed and irrefutable information about the context
of the music, how it relates to other pieces at the time, and how this correlates to the aesthetics of the available interpretive choices. Notable is his humility and kindness in conducting the class (in contrast to the pompous and bland John O'Conor masterclasses which I also recently came across). Even if you disagree with him, you'll still come away with new insight to the music.