Well... Seeing as I'm already terrified of zombies and other undead creatures, I spent the first six months from when I started to play Skyrim, persistently avoiding nordic tombs and similar places if I was home alone (even in daytime, yes). I cope better with it now, but... I still hate the damn things. So top on my list will be...
1. Draugr - add a slightly different texture, like some blood, a little darker skin, and have them scream at you some more. That'll make me stay far away from your mod. I hate being scared.
2. Chaurus Reaper - because yes.
3. Dragon Priests - Again, I think it's the undead thing... Also maybe that they kick my ass.
Edit: I'm not counding any DLC creatures here, as I have only been to Solstheim very shortly and I haven't come far in Dawnguard questline either, but... those ash thingies I got to see during my short visit to Solstheim would definitely make it on my list.