Miss the most: Rikti
I don't know why but there was something so satisfying about killing them and watching them crumple to the floor. Bonus points if the last hit had KB so they went flying into a wall, limbs flailing in that hilarious way that only ragdoll physics can deliver.
Miss the least: Malta/Knives of Artemis
They definitely lived up to their role as an endgame enemy group. They could debuff you into the ground, chain CC you to death, and put out serious damage. That's not even including the damn Sappers. Two hits from one of them and your blue bar was completely empty. Just like that, you've got no offense OR defense and the red numbers just came rolling over you until you either popped an inspiration or faceplanted.
KoA wasn't nearly as dangerous but every. single. mob had caltrops. Especially in tight office maps, if you couldn't fly you might as well just get used to running in slow motion every fight. The damn things stuck around after the mob that set them up died too so just killing them wasn't enough to keep it from happening.
Special mention to The Council for being the most boring, homogeneous group in the game. Most of them were literally just recolors of each other with an extra power or two tossed in. I did enjoy the "chuffchuffchuff" sound their Gatling guns made though. Really made fights with them feel like mass shootouts with bullets flying all over the place.