Sweet = Sugar, Salty = Salt, Sour =?

Sour=acid

Weak organic acids (e.g., the acetic acid in vinegar) taste more sour than mineral acid at the same pH. according to a review at Elsevier's Science Direct topic overview.

So your substances are the culinary acids: citric, acetic (vinegar), malic, tartaric, fumaric. Of those, acetic acid is the simplest molecule.

what is the simplest substance that makes this taste like that

The problem with this sentence is the definition of "simple". What you are describing in your examples is not "the simplest" substance for a taste, it is the prototypical substance for a taste - the one which we have encountered the most commonly, and our brain associates most strongly with the term for the taste. So, the only sense it is "simple" is from the point of view of gestalt psychology, not any objective quality of the molecule.

That being cleared up, here an explanation of the tastes and what they are meant to denote:

• the sensation of "sweet" means that your mouth has recognized simple carbohydrates (simple in the chemical sense, typically monomers or dimers as opposed to polymers like starch). The prototypical sweet substance for our culture is sucrose, as you already noted. This is despite there being other substances which have a stronger sweet taste (e.g. saccharine) or being chemically simpler (e.g. glucose).
• the sensation of "salty" is a recognition of chloride ions. The prototypical substance is indeed table salt.
• the sensation of "sour" discovers acids, as others have noted. For a typical Western culture, the prototypical acid will be either acetic acid or a mixture of citric and ascorbic acid, depending on whether your family tends to use more vinegar or more lemon juice.
• the primary purpose for the sensation of "bitter" is to discover alkaloids. There is a huge variety of alkaloids, and humans have been breeding them systematically out of their food plants for centuries. Thus, there is no single prototypical alkaloid which everybody uses as the standard for bitterness. A potential candidate would be quinine (in tonic water), but we encounter many other bitter substances with a similar (low) frequency. Also, we do encounter bitter tastes in other foods, especially when there are metal ions present, but these tend to be perceived as less bitter than alkaloids.
• the sensation of "umami" discovers glutamates. The prototypical substance is monosodium glutamate, MSG.

---

For completeness of sensations felt with the tongue (despite not being traditionally counted as "a taste"):

• hot/spicy is a type of pain produced by capsaicin or mustard-like substances corroding your mucosa
• astringent is a sensation caused when tannins cross-link proteins on your cell surface and in your saliva, changing the configuration of the cell surface and reducing the saliva lubrication
• cold is a sensation whose mechanism consists of triggering the TRPM8 receptor. This usually happens by actual coldness (e.g. eating ice cream) but accidentally, it so happens that menthol also activates this receptor.