Sweet = Sugar, Salty = Salt, Sour =?
Sour flavours come from acids, like citric acid (in lemon juice, for example) or acetic acid (in vinegar). I don't think there's any one acid that qualifies as 'simplest'.
Bitterness is much more complicated; there are lots of different foods (coffee, uncured olives, citrus peel, alcohol, hops quinine) which are bitter to some extent but there isn't an obvious culinary ingredient/chemical that just gives 'bitter' as a taste. Our perception of bitterness is believed to have evolved to signal toxic foods.
It's worth pointing out that your existing examples are an oversimplification: C₆H₁₂O₆ is glucose, which is only one kind of sugar, and there are others which also taste sweet to us (as well as non-sugar sweeteners, some of which taste far sweeter by mass than sugar. There are also other salts, like KCl (potassium chloride). There is much more information in the Wikipedia article if you are interested.