If by “slow cooked” you mean cooked long and slow for hours on end so the middle is overly done, I would say many steaks (not all, though), pork loin, and chicken breast. Some people like chicken breast and loin cooked for a long time in a slow cooker. I find the texture yucky, no matter how much sauce you try to cover it up with.
The best salmon I have ever had in my life was on the Makah Indian reservation at the northwest tip of the 48 contiguous states, where the cooks built a big fire, filleted whole salmons, and put the salmons, flesh-side in, on stakes facing the fire. It was amazing.
(Actually, to answer my own question, I rember not that long ago cooking a smoked sausage sauce in a slow cooker - it turned out absolutely disgusting)
A number of people I trust are convinced that sous vide prepared steaks are the finest preparation. I’m not really a steak guy and I’ve never had a sous vide one anyway so I couldn’t say. I’m not sure seafood would work, either.
I disagree with this. Brisket tastes better slow cooked or barbecued (slow-cooking with smoke). I would say the same for tough cuts of meat like pork shoulder and chuck, although the grilled versions of those, if sliced really thinly, are pretty darned good eats. And while short ribs are quite nice grilled, they really shine when they are braised.
The problem with the lean cuts of meat that don’t have a lot of collagen in them is that slow cooking to a well-done state doesn’t do anything of benefit. A filet, a loin, a breast, etc., just dries out when cooked for very long–it doesn’t get any softer or juicier or anything.
ETA: And, to be clear, by slow cooking I mean braising or using a Crockpot or whatever, where you cook low and slow for hours and hours to break down the collagen and soften a piece of meat. I am not including sous vide or similar styles of slow cooking where you don’t cook the meat to 170+ degrees.
I don’t know, some bacon-containing recipes are fairly slow cooked. Candied bacon and bacon-loaded baked beans come to mine. I particularly enjoy low & slow bacon wrapped, cheeseor sausage stuffed jalapenos on the smoker.