One major reason for CK being the GOAT is that it was the first to pretty much establish what we have known as film grammar to this date. That's not to say it was necessarily the first to do things like deep focus, etc. But it was the first to do it intentionally as a storytelling technique. As it was said before, watch the Ebert commentary. I had already seen the film many times and coming from film school, had already read about it and discussed it many times by the time I saw the commentary, and I still learned things I never knew.
As for the entertaining aspect, I completely agree that a film should be entertaining. However, entertaining is really a completely subjective idea. That's what makes trying to rank films pretty much impossible. But I do think CK is very entertaining and I don't find it dull at all. So, yeah, on my list, it's pretty much between Kane and Goodfellas. But I named my dog Kane.