What is the most common animal mistreated?

Neglect and cruelty are the most common abuses of animals. Both abuses cover a lot of territory.

You might think of neglect as those things withheld from the animal - food, water, proper shelter, vet care, attention, etc. Such situations are frequently due to a lack of understanding the animal's needs so proper care is not provided. This can often be remedied through education.

Cruelty is those things intentionally done to an animal - physical abuse like hitting, choking, burning, cutting, etc. - that physically harm or injure the animal.

Another opinion:

Probably tying the animals mouth shut, legs together, etc. or leaving the animal in a bag to die. Also just hitting an animal for no reason.