Prometheans. Though to be fair, they were vastly improved between H4 and H5. Regardless, in both iterations the Prometheans were bullet sponges. The Knights were too OP in H4 with ridiculous extra abilities like teleportation that allowed them to escape death or deliver death instantaneously. The Watchers were also OP because they could block grenades, provide cover, and resurrect allies. Not to mention, it took way too many shots to kill them and they were so small that you had to be precise with each shot in quick succession. The Crawlers weren't OP, but there were so many of them that they ate up your bullets. And since H4 had such scarce ammo, each bullet mattered.
The Flood on the other hand didn't have ridiculous abilities like Knights and didn't have the ability to block grenades or raise shields (though they did resurrect themselves if you didn't kill the Infection Forms) like Watchers. You could take them down with any number of weapons in any number of ways, which made game play more interesting imo.