Sourness is subjective, therefore different people may have varying opinions on which candy is the most sour. A reviewer from Candy Addict feels that Toxic Waste candy is the most sour.
Malic, lactic and citric acids are the three ingredients most commonly used to give candy a sour flavor. Of these three, malic acid is the most potent.
Warheads are a popular candy that many people find to be quite sour and intense. Warheads are especially sour because they are coated with malic acid. Many other sour candies are made only with citric and lactic acids instead, which are more mild. Once a person eats the malic acid coating from a Warhead, a sweet-and-sour interior is revealed, containing citric acid.
Toxic Waste candies are often considered more sour than Warheads, since they have a sour core. They are comparable to Warheads, except for this additional sour center.