Onions. Everything about them is repulsive. Their scent, their consistency when they're not cooked to the point of being soft, the taste itself, etc
Way too strong.
Whoever found these things, decided have a taste, then brought it over to civilization is a horrible person.
It doesn't help that they're in so many dishes, you can't really escape them unless you just stick to dessert for the rest of your life.
I think the only way I can have them without getting sick is in soup. Since it ends up soft and the flavor is evenly distributed, it poses no issue.
Aside from onions, there are also mushrooms, tomatoes, olives, lettuce (cabbage is yum though), chicken, cottage cheese, shellfish, any fish that isn't mahi mahi, anything even mildly spicy, tofu, bananas, apples, fruit juice, cherries, peppers, shredded coconut, peaches, celery, beans, bacon, liquorice, and the list goes on.
I'm not picky. I'm just against anything that tastes, smells, or feels horrible.