Myself, I can't eat onion, tomato, peppers or banana.
Correctly processed I can, but straight up, no. I like the taste onion gives to something, but biting into a piece is extremely unpleasant. So, no onion on pizza or fried. Stewed, in sauces, etc. is usually fine. It's not just the strength of the flavor either since I like the taste of garlic. As in: biting into a garlic clove, though preferably baked.
Tomato, I just don't like. Ketchup and sauces with it, I do.
Like salsa, can't stand bell peppers. I like jalapenos, but have no spice tolerance. Go figure.
Banana makes me sick, but diluted into banana bread is delicious.
Worst I've ever personally experienced was a friend of mine in high school. Near as I can figure, the family lived on a specific pasta and nothing but. I know that is father adored pasta and they had it often, but the guy wound up staying with my family for a week once and my parents nearly kicked him out after a few days.
Fast food was okay, but almost literally anything else we gave him we had to work to get him to even try. When he did, he looked like he just licked a slug. He had no allergies, but I remember him refusing to eat tuna, scrambled eggs, chicken baked in tomato sauce (not my favourite, but I can eat it happily enough), scalloped potatoes, mashed potatoes, hamburger patty with brown gravy, and spaghetti made with a different sauce than he was used to (homemade tomato and ground beef).