I guess whether fears are irrational or justified is all in the outcome. If I conquer my fear of swimming in the ocean; that's a victory. If I'm subsequently eaten by a shark then that fear is proven well founded.
In that respect we each only have one thing to rationally fear; whatever ends up killing us.
Since we often can't know that in advance, odds are very slim that the thing you're scared of is the thing you should be scared of. I suppose fearing death itself is a safe bet; but then that's kind of pointless because it's inevitable.
I wonder how many people who fear dying of something actually die from that thing. It could be that the fear has helped them avoid it, or it could be that the chances are so slim that it wasn't likely to happen anyway. People fear dying from terrorism, many more people die in home accidents.