According to Zeddicus, all small dogs for sure. Chihuahua, Doxin, Pomeranian, Miniature Pincher, Boston Terrier or just to name a few. Dog owners tend to spoil small dogs. This is why they often get a pass and have been getting away with murder.
Small breeds bark nonstop, nip at you, lunge, snap, not reliable off leash and are rarely 100% putty trained. Probably the only toy breed that is easy is toy or miniature poodle. Oh, and maybe Maltese and Papillion.

Among big dogs, here are Dog Prodigy’s top list:

Pitbull, Akita, Doberman, Chow, Rottweiler and Bulldogs. Whether you own an American bulldog, English or French, they are all very stubborn and hard to train.

Pitbull’s or Pit Mixes: Sorry but these dogs have a bad reputation for a reason. They are unpredictable with strangers or with dogs. They are true gladiators and most aren’t social let alone fully trained. They attack to kill. Even if you own a Pit mix, you HAVE to make sure that it is far more trained
and polite than people’s kids. Shelters are full of pits and pit mixtures. Sadly, they label them as shepherd or lab mixes so they can be adopted.

Labrador and Golden Retrievers: Yes, both breeds can be very friendly but also super strong, hyper, easily- distracted. These two breeds rarely get protective, but can drag you like a mule on leash and aren’t always reliable
once they find themselves loose. Especially once they spot another person, cat, jogger, dog or when you have company over.

German Shepherds: Ah yesses. Zedd’s favorite breed. A dog of dogs. But they tend to get protective very fast. They start to get weird towards dogs or men. Also very tough and stubborn. So intelligence isn’t always enough. Labradoodles or Goldendoodles: Very cute, smart but stubborn, hyper, easily distracted and love to run around playing keep away with you. The poodle made these dogs highly intelligent which sort of backfired. Because they find ways to outsmart their lovely owners.

Siberian Husky/Alaskan Malamutes: Love to pull you and they are escape artists and terrible in coming back to you. This breed is gorgeous but make lousy guard dogs. Once these dogs bolt, don’t even bother going on foot looking for them. Get the car and ask your entire family and neighbors to help you.

Became more popular after people fell in love with the wolves of Game of Thrones. Unfortunately, once people realized how much work it is to own and train one a Husky or an Alaskan Malamute, they start dumping them in shelters. Very hyper, noisy and strong dog. They also dig, shed and dump their water because California is too hot for them. If the word terrier comes after the dog, that is one stubborn dog. Any terrier. Pitbull terrier, rat terrier,
boston terrier. If we have to make our 2 nd most hard-t-train dogs it would be hounds.