Contrary to a popular southern tall tale, the granddaddy long legs (Opilionids) is not the most venomous spider in the world. It isn't even a spider, and it doesn't even have venom glands.
However it's look alike, the cellar spider (Pholcidae), is a spider, does have venom glands, and can bite a human, but their venom does not have an adverse effect on a human.
So if you ever hear someone telling you that the granddaddy long legs is the most venomous spider in the world, one bite could kill a human, but their fangs are too small to bite a human... you can now inform them that they are incorrect