After cooking some might taste funny. There have to be some where the texture suffers from cooking to the point where you wouldn’t want it. Perhaps there is a rare food that is unhealthy to eat after cooking.
Spinach. Great raw, disgusting when cooked.
For me it’s Tuna
I love tuna sushi and sashimi. And I also love the seared tuna appetizers where the outside is quickly seared but the inside still raw. I hate fully cooked tuna though, at least as standalone meat. It’s fine if it’s added to some other recipe like Tuna Salad but that’s because the “cooked tuna taste” I dislike is easily covered with cream and mayonaise.
I have the completely opposite take. I hate raw spinach, love it cooked.
My attempt - radishes. I like cooked radishes just fine, but honestly I don’t think it improves them any. Raw radishes are as or more tasty and I might venture to say more versatile as well. I’d rather raw radish than cooked in my carnitas taco.
Avocados are pretty much ruined by cooking. I don’t know of any recipes or classic dishes that call for cooking them.
I’ve never seen or heard of cooked melons such as cantaloupes or honeydews. I have had watermelon jelly, which I guess counts as a sort of cooking.
My wife stir fries leaf lettuce sometimes. I would much rather have it in a salad; it’s kind of bitter and soggy when cooked.
I’m sort of middle ground. I can enjoy a spinach salad just fine, but truly love love love spinach sauteed in olive oil, butter, and garlic and then lightly salted with lemon juice drizzled on it. I’ll ignore the entree at times, for that.
As to the thread topic, I too love my tuna raw, but my enjoyment goes down the more it is cooked.
There’s a type of lettuce that’s used in Chinese cooking. It has more stem and less leaf than the lettuces usually used in salads. I’ve seen it sold in seed catalogs as “celtuce.”
Apples. I’ll eat apple pie, but the texture is never pleasing to me; plus, you gotta load up on the cinnamon and sugar. Applesauce is lame, taste-wise. Baked apples don’t cut it. It’s almost like apples weren’t meant to be anything but raw.
I gotta go before those trees start throwing apples at me.
For me it’s carrots. Raw carrots are okay (not particularly great, but okay). Cooked carrots are gross.
Most vegetables, to my taste, are best either raw or only very slightly cooked. Certainly, any vegetable is ruined by overcooking.
Though I’m not thinking of any that are no good when cooked at all.
Cookie dough.
100% in agreement. Otoro is among the most enjoyable sushi. But just for something different, sushi bars will sometimes sear tuna with a blowtorch. If grilling tuna, the appropriate procedure is a very hot grill for a very short time – it’s searing you’re after.
If tuna is not of sufficient quality that it can be consumed raw or rare, give it to the cat. 
Oranges. Sure you can make a stewed orange-in-spiced-wine compote or something of the sort, or candied orange peel, but nothing will match a high quality fresh orange.
Grapes (excluding “cooking” into wine). I have a grape and cranberry pie recipe that’s actually pretty tasty, but if I had to choose between that pie v. raw grapes for the rest of my life, the raw grapes would win hands down.
I’ve never heard of cooking dragon fruit, but I can’t imagine it would end well.
Very true. I’ve tried recipes for fried avocado (intended for when you accidentally cut into your avocado and find it isn’t as ripe as you’d hoped) and occasionally thrown some leftover avocado into something I was cooking. Based on my experience, I will say: no, just don’t do it. If the avocado isn’t good as is, cooking won’t help. If it IS good as is, then what the hell are you cooking it for?
Most fruit is better fresh than cooked. Exception that I can think of off the top of my head: cranberries. Maybe bananas (because I hate bananas, but banana bread is tolerable). Otherwise, whether baked in a pie or a cake or made into compote, cooked fruit just loses something compared to fresh. And that’s just the flavor. How many of the nutrients are lost from cooking fruit? (I don’t know, but I’ll bet it’s a significant amount).
Sweet potatoes taste pretty good raw, like a crunchy milky-sweet carrot. The flavor changes very much for the worse when cooked. Carrots used to do the same thing when I was young, but now I like them cooked. Celery doesn’t really taste worse to me when cooked, but it does seem to take on a stronger flavor after cooking.
Salmon. Good salmon sashimi is one of the best foods there is. It has a texture I describe as meaty butter. It’s smooth, buttery, soft, and meaty, all at the same time. The flavor is very mild, and not at all fishy. Cooking adds a fishy flavor and takes away the buttery softness.
Nothing with dragon fruit ends well. It’s like Yum Kaax looked down upon the nascent spirit of the dragon fruit at the dawn of creation and decreed it would be the most amazing-looking of all fruits, but to balance it out it would be drained of all discernible flavor.
I guess at least it did better than jicama, which is both ugly AND virtually flavorless 
.
Asparagus is one where it needs to be cooked a bit. Raw asparagus is strongly vegetal, like grass. A touch of heat knocks that down.
Overcooking it to mush makes it worthless, of course.
Unless you’re grilling it or broiling it so it gets a little dry and crispy. Asparagus roasted with crunchy brown touches is very nice.
Yeah, grilled asparagus are wonderful.
I don’t care for most cooked fruit, outside of desserts. I agree with what was said upthread about apples, but they’re part of a sauce you can make with a specific kind of non-spicy pepper, red onion, garlic, white wine and bread crumb (not bread crumbs). The apple takes the edge off the aggressive taste of the other ingredients and makes the sauce smooth and tangy.