The saddest thing that can happen to a progressive musician is a conservative fanbase. There is nothing wrong with bands who keep a consistent style and try to record the same but different album over and over. They often say they know what their fans are looking for and don't wish to subvert their expectations. And there is a great value in that. Obviously, in such cases, the first couple of albums are usually the best ones. 

But King Crimson was never that band. Always on the frontier of musical groundbreaking, experimentation with different lineups and different instrumentation, and different sounds. To say their first album is the best is not only disappointing but even disrespectful to some degree. A musician, who is constantly evolving, developing, searching, and learning, should be valued for all the work done through all those years. 

I believe that the last album of King Crimson is the best. Not only because it is basically a progressive metal. It sounds most polished and most cohesive to my ears, like a true summary of decades of amazing music. It has everything from a cappella to electronic, and it even has an amazing story. Some even say that you can find Tool inspirations there. When someone who helped to create something goes back to hear what younger generations who followed in his footsteps have to say, that is a sign of a true master.