Unfortunately for rats they taste rather good. They are far down the food chain, meaning that a large number of animals happily eat them. Large predators such as snakes may eat a whole rat in one go whereas smaller animals may eat them gradually a bit at a time. They may not be the first choice meal for many but they will certainly do if no other food source is available.
Weasels. These rodents can be very aggressive and will happily eat a rat.
Large mammals. Coyotes are relatives of the wolf and dog and will also eat rats. Foxes eat rodents as well as vegetation such as grass and berries. Some large cats such as mountain lions usually go for larger animals but will happily eat small rodents and birds if nothing else is available.
Snakes. The rat snake predominantly eats rats and favors them over other food. They don't usually harm humans so are useful for controlling rat populations. This type of snake is common in urban areas. They frequently live in disused outbuildings. The boa will swallow it's dinner whole and larger ones can eat a whole monkey, pig or deer. The rat snake and the boa will both suffocate their quarry by squeezing it with their powerful muscles before eating it.
Birds. The most commonly seen hawk in North America is the red-tailed hawk. They are often seen in urban areas flying over open spaces and sitting on telegraph poles. They have excellent vision which gives them the advantage when it comes to spotting and catching rats. The barn owl also enjoys eating rats and will hunt them at night. They fly very quietly and have acute hearing which helps them track down their prey. The northern harrier has a face like an owl and is a variety of hawk. It will fly close to the ground and happily eats rats. Interestingly this species of bird may drown it's prey in a nearby body of water before eating it.
Humans. People all around the world eat rats. In the southern parts of the USA a large variety of rat called nutria is often eaten Cajun-style or grilled. In the Indian Mishimi culture the women are only allowed to eat pork, fish, rats and wild birds (!). Rats were a traditional food in Polynesian cultures. In New Zealand the Maori were very protective of their rats and outsiders needed to ask tribal leaders for permission to hunt them.