People are often confused about what makes a beer sour. Beers that use only a Brett and a Sacc strain are typically not sour, but can be a little tart. Brett provides, barnyard, leather, horse blanket, pineapple, and cherry pie qualities depending on the strain(s) used.
Lactobacillus and Pediococcus are what makes a beer really sour. Brett is often used in beers with lacto and pedio. A beer like Orval that uses Sacc and Brett B is not a sour beer.
The sourest beers are from Cascade, Russian River, The Bruery, Cantillon and Drie Fonteinen. Jolly Pumpkin's beers are sour, but not nearly as sour as the aforementioned brands.