Horrific (IMO): Insect, twisted, blood, toxic.
Ease of bringing into the game: Blood, twisted, toxic, insect.
Likelihood of running across: Twisted, toxic, blood, insect.
Blood mages: 'He turns, cuts the bystander nearest him across the throat, and blows apart the van behind which you're taking cover. Oh, wait, that's Joe's van? Sorry Joe.' Easy to pop into the game, a sudden burst of 'oh crap' scary, and can be very long-term scary as you build it up. Not actually too frequent in the game world.
Toxic mages: 'Geez, this place is nasty. Are you sure the guy we want is hiding heeeaaaaarrrgeezrun!!' You generally know that you're going into a nasty zone, and a glance on the astral can tell if it's toxic or not. Gets scary when they go out trying to make more toxic zone (or commit murder on an industrial scale). Considering toxic zones (and types of toxic mages), far more frequent in the game world than anyone wants to admit.
Insect spirit/mage: "AAAAAAAA!! Where's the flamethrower -- where'd you put the -- AAAAAAAIIIIGGGGHHH!!!" Can be a wonderful, horrible combination of 'Body Snatchers' and 'Alien', what with excellent merges and full-bore flesh forms. Usually the sort of thing you need to build up to; not something often stumbled over, so this is the sort of thing where a plan is required.
Twisted mages: "Oh my god. What ... what happened ... these were my neighbors, I was gone two frickin' days -- get out of my way, I'm gonna *BLORT*" Semi-toxic, semi-blood-type, warped, twisted ... shadow spirits love 'em, Son of Sam sort of serial killer stuff. The attitude of blood magic, the power acquisition of toxic mages, and a total 115% whack-job spin at the end ...