I don't know if I'd be so quick to call it the best because I don't think I have a static opinion about what content is my favorite- heroic vs. epic/legendary, my build, and what I'm hoping to get from the pack all significantly impact what I would choose to run and how I compare packs, and it's definitely too early for me to weigh in too much on Demogorgon as it relates to other packs.
I will say that I have enjoyed Demogorgon a lot, there's nothing from Demogorgon that I would point out as "bad" (though there are some parts I enjoy less), and that I think it has some of the best environmental design in a long time- far better from a technical/aesthetic standpoint than Ravenloft in my opinion. But at the same time it's not perfect, and there is probably a bit of "new and fresh" vs. "old and familiar" going on too- I'll probably have different feelings about Demogorgon when it's almost ten years old too.