* Ammonia is a base, not an acid.

* Coca-Cola is mildly acidic due to the presence of phosphoric acid.

* Vinegar is also acidic, primarily due to acetic acid.

* Drain cleaner can be highly acidic, often containing strong acids like sulfuric acid or hydrochloric acid.

Therefore, drain cleaner is the most acidic of the listed liquids.

It's important to note that "most acidic" doesn't necessarily mean "strongest." The strength of an acid depends on how readily it donates hydrogen ions (H+). Drain cleaners contain strong acids, but their concentration might vary.

Always handle acids with caution and follow safety instructions.