Being serious for a moment, my greatest fear is that my mother or my wife will outlive me.
I have HepC and a pretty shitty prognosis (not fishing for sympathy here, I've known this for about a decade and it is what it is).
All I really want out of life is to live long enough to support my mother until she's gone and to not leave her or my wife high and dry. Luckily, considering my health circumstances, I've got a killer life insurance policy from my job (twice my annual income in a lump sum if/when I die) and my medical prognosis says I have a good 15 to 20 years left - if I take care of myself (which I am doing) and that should be enough to outlast my mother.....I hate to even look at it that way, but I just want her to have a comfortable life in her golden years......I put her through hell until about 15 years ago, so I've got a lot of making up to do.
Anyway, that's my greatest fear - dying before my mother and/or my wife.