The Splicers in Bioshock are my favorite. Technically they aren't monsters (they're superpowered junkies), but you mentioned Halo's regular enemies too alongside the Flood so I guess I can talk about them.
The Splicers have cool designs, and they could be intimidating or even funny depending on their level of craziness. The basic ones use guns and other melee weapons, while the other variants have their own unique movesets (fire, bombs, etc.). Taking on an individual Splicer is easy, but when they gang up on you it's a different story especially on higher difficulties.
The teleporting ones are pain in the ass. Sometimes you'll waste some ammo on them, and when you try to get closer to hit them with the wrench they disappear.
The bomb-carrying ones are annoying too. They throw bombs while running away like chicken, but you can use telekinesis to make the fight easier.
The Spider variant is the most interesting of the bunch. The way they stick to walls and move around using their hooks was really creepy in 2007. They fight using these hooks, and they can throw them at your face. I imagine they would be scary in VR.
You know.. If they're really making a new Bioshock, I personally hope they go back to Rapture. I liked the enemies in Infinite, but Splicers are more interesting and they could be made even better using today's AI tech.
