I would say the opposite. The first book is good, but pretty typical sci-fi fantasy fare. The next books are what really fleshes out a really interesting concep

I would say the opposite. The first book is good, but pretty typical sci-fi fantasy fare. The next books are what really fleshes out a really interesting concept, this universe where humanity is far more advanced than today in a humanist way instead of technologically. They also help make the point of the first book clear - that Paul wasn't a hero, but a tyrant. This is easily lost in the first book because of the comparison with the Harkonen and the previous Emperor, and because we're used to rooting for the main character. But Herbert's point wasn't to further the myth of the philosopher king, but to dismantle it - which is quite rare in fiction, and made extremely clear throughout the series. I'd also say that the exploration of Leto II's psychology in the God Emperor of Dune was the most interesting writing in the series, so that's what I'd consider the pretty clear high point.