Blood/YMMV

Complete Monster: Practically EVERYONE. Caleb, the Cabal, the Otherworld creatures, Tchernobog, the Ancient One...
Crosses the Line Twice: The game's main selling point.
Genius Bonus: The weird, creepy language of the cultists is actually a mixture of Latin and Sanskrit words and grammar. Check it out.
Goddamned Bats: Definitely. Also rats, spiders, choking hands, and the Bone Leeches in Blood II. The Choking Hands and Bone Leeches enter Demonic Spiders territory if you don't know how to get them off. It takes nerves of steel to fight them without flinching.
Good Bad Bugs: The first chapter of Blood II has you visit a laundromat twice, as noted in Copy and Paste Environments. With noclip, it's possible to take the exit you're meant to use the second time through and skip at least two levels. The first game has the "flame guy" bug, as noted for Dead Character Walking. Also in the first game, the Merry-Go-Pukey in E1M4 Dark Carnival will spawn gibs if it's attacked, and the Bloated Butcher's cleaver throw can be negated by crouching (the projectile hits, but doesn't cause any damage).
Hilarious in Hindsight: Blood 2 has an Easter Egg on one map which is a hidden Predator lair, including a couple of xenomorph skulls. Three years later, Monolith made Alien vs Predator 2.
Most Wonderful Sound:
- The same cultists when set on fire. They may be annoying normally, but their girly, undignified screaming and character-breaking English shouts of "It burns, it burns!!" more than make up for it.
- Caleb's five variations of Evil Laugh when you blow something up, going from quiet chuckles to full-on psychotic cackling.