Cyrus D. 
Day 8 - Favortie Federation Enemy - The Dominion
Almost said the Borg as they were just so terrifying at first. But Dominion is better fleshed out. The reasoning that leads the Founders to go to war more nuanced, and of course the way the war ultimately develops. I have no basis for it, but the Founders and their relatant plot arcs feel like the continuation of what was originally conceived of with the parasite aliens from the TNG Season 1 episode “Conspiracy”
