Mine is "The Beyond". But Gates of Hell is second, House by the Cemetary third, Zombi forth, then House of Clock and Manhatten Baby. Anyone else a FULCI fan? He's my all time favorite Director and inspiration for me to keep writing horror stories.
"Zombi 2" (Zombie) for sure is my favorite.
Followed by "City of the Living Dead" (aka Gates of Hell), and "The Beyond". I'm rather fond of "House by the Cemetary," though it seems to be panned by most.
I hear good things of "Don't Toture a Duckling", but really I don't remember much from the one time I've seen it; it's surely worth a repeat viewing then.