My biggest fear since I stopped believing in hell is that when I die I'm gonna be fully aware of the darkness and isolation instead of it being like going to sleep.
Being helpless and totally dependent on others.
Spiders, heights, and living life without anyone I can truly rely on or trust. Oh and the dying alone thing, but others have said that.
It use to be being the very real likelyhood of being shot by north Vietnamese now it's the only slightly less likely chance of sitting in a class room or concert or gathering of any kind and some asshole "law abiding" gun owner walking down the hall/street with an ar-15.
Because of the former I avoid the later.
It used to be the boogeyman in the closet, till the monster under my bed came and chased him away. Seriously though, it's dying a long slow painful death, like from cancer or a stroke. I watched my grandmother, grandfather, great grandmother and mother go that way. Fortunately I've had a heart attack and will probably go quickly.
I have a slightly irrational fear of fire. I'm a big furball and I always imagine going up like a match when I'm around fire I don't have control of.