It kind of depends what you call the worst, like a few others here I often have bad spontaneous nosebleeds. Aside for the boring long ones I have about three that could classify as worst in my opinion.
The time I got one on my bicycle on my way to school with nothing to stop it. Rushed to the closest house, though it was pretty far away. After about half an hour I arrived, my vest and shirt were fully blood soaked and it took nearly as long to get the blood out of my sleeves.
I once had one on April fools, no one believed anyone could have one that bad. And once again, I did not have anything with me or close by.
And then there is the time that I nearly fainted from blood loss while in bath with the door closed, the water was pretty red.