Actually, humans and other mammals have some of the strongest stomach acid around.

Our stomach produces hydrochloric acid (HCL), which is very strong. However, its not our stomach acid that breaks down food in the stomach. The acid just allows another chemical, called pepsin, to work properly and break down food. Pepsin is a digestive enzyme. Other digestive enzymes are found in you saliva and intestine.

The stomach and its chemicals are just part of the digestion story. Most animals have three ways to break down food to get its energy: mechanical,chemical, and bacterial. Chewing is a mechanical way to break down food. The digestive enzymes in saliva, the stomach, and the intestine chemically break down food. Helpful, natural, bacteria in the intestine also assist in digestion. What animals can digest depends on all three, so stomach acid is really the least of the story.

For example, humans cannot digest cellulose, also called fiber, found in plants. Cows have intestinal bacteria that can break down cellulose so that the cow can use it for food. They also chew their food multiple times to break it down. So it's not the stomach acid that allows the cow to have a more powerful digestive system.

Venomous snakes swallow their prey whole and depend entirely on digestive enzymes to break down their food. Their venom has digestive enzymes in it to start digesting prey before it is even swallowed. They have more powerful enzymes, not stronger acid. They still cant digest bones and other hard parts.

I think the most impressive stomach would probably belong to the white shark. They have huge stomachs with very powerful digestive enzymes. They eat their prey in large chunks. Because great white sharks swim almost constantly, they need to digest quickly. Their stomachs turn prey into liquid very rapidly. Their intestine has digestive enzymes that further break down food. As a result they can almost completely digest the seals and other mammals they feed on.