There are many houses and other groups in the novels. The most well-known ones being House Atreides, House Harkonnen, the Bene Gesserit, and the Fremen. But Frank Herbert’s created many other groups in his fictional universe, some of which gradually evolved, and some which just died out.
Which is the one you are most fascinated with? And why do you find them so interesting?
Choosing a most fascinating group is like choosing a favorite ice cream—I can’t do it. All are fascinating for a multitude of reasons, many of which have been touched on in the comments so far. This is one of the many reasons why these books have stood the test of time. The social groups, along with the worlds and universe are just so well developed with plenty of parallels to the human race and ethnic groups on Earth.