The toughest meat to cook is Brisket.
Brisket is the toughest meat to cook, because of it's large size as well as tough connective tissues also known as collagen, which require long, slow cooking.
The challenge in cooking Brisket also involves cooking it's 2 different muscles evenly without drying it out.
Brisket is a cut of meat that comes from a heavily used muscle in the animal and is packed collagen, which requires hours of low and slow cooking to break down the collagen into gelatin, otherwise the brisket will be tough and chewy.
Large cuts of meats like brisket with muscle fibers running in different directions make even cooking and timing of cooking brisket and other similar meats incredibly challenging to cook properly.
And brisket demands several hours of cooking for tender results so it's not a quick meal.
Other toughest meats to cook are fatty Wagyu which is easily overcooked, lean cuts of meat like Rump and delicate protein meats like Sweetbreads or any meats that require a crispy skin with tender meat, such as duck meat.
Wagyu (A5) has high fat content, which melts quickly and makes it easy to overcook and turn oily, which also requires a different approach in cooking than regular beef.
Rump steak is lean with tough and long muscle fibers and sinew, which require careful trimming and preparation.
Sweetbreads like Thymus/Pancreas require extensive and time consuming preparation and can also become tough very easily.
And even Game birds meat and Pekin Duck are tough to cook as achieving a crisp skin and tender and non dry meat simultaneously is a common challenge when cooking these meats.
Even T-Bone and Bone In Cuts of meat are tough to cook as well as the bone insulates the meat, and cooking the 2 sides such as the strip and tenderloin at different rates, makes getting them done tricky.
Lean cuts of meat like Shank and Rump lack fat, which make these meats prone to drying out.
And brisket in particular usually cooks differently each time.
Seafood can also be hard or tough to cook, especially octopus.