Of course, the Illuminati didn't really change over the course of the story, since the whole arc was about their slide from kings into failure.
But, my take is that BP had the most up and down changes, while the rest of the Illuminati static.
BP and Namor both show some development, due to the nature of their story and conflict. BP starts off moralistic, then pragmatic when he opposes Cap, then shows resolve in blowing up a planet, then loses everything, then shows an inability to compromise his morals to blow up a planet, then becomes vengeful, and then finally still works together with Namor after truly losing everything. Namor has a similar amount of change to arrive at the same point with working with BP.
As for some of the other Illuminati: Cap is stubbornly moralistic while Tony is stubbornly pragmatic at the beginning, and both remains that way to the end. There is no moment of personal reflection for either character, they continue being the same way. Even Tony, when he is imprisoned, continues to rant that he would not change anything. Beast and Prof X had little screentime for any type of development. Black Bolt was there just to represent an already fallen king. Reed doesn't change for most of the story (he remains pragmatically optimistic) until the end fight with Doom where he describes a changed outlook. So essentially, they all slide into failure while staying the same.
I do agree that it is personal taste on what is most interesting.