While I could probably play every campaign a million times and not get tired of them, I've got to admit some of them just have mechanics that've made me super annoyed, and others feel like a breeze.
I personally love Dead Air and Dark Carnival, especially their final stages. But honestly smth about Blood Harvest and Hard Rain make me so tense every time I have to play them. The corn field on Blood Harvest is always such a pain to get through unless you have a competent group, and Hard Rain has both the return levels which are just, nuts, and the disgusting amount of witches just wandering around having seperate breakdowns.