Being a brewer, I guess I have a different wiew on «bad beer» than a lot of people.
I like all styles of beer, provided they are technically well brewed. I can enjoy really bland beers like Budweiser, Bud Light, Coors, Corona etc. because they are technically EXCELLENT products. They don’t have tons of flavor, but they don’t have any defects either. (Except maybe Corona which quite often is lightstruck and skunky because of the clear bottles, but that’s not the brewer’s fault.) Big industrial brewers have a technical consistency that is simply amazing. I don’t often drink stuff like this, but it is always interesting from a brewing perspective.
When I think of bad beer, I always think of technically flawed products, i.e. beers suffering from bacterial infections, excessively high fermentation temperatures, over carbonation, unbalanced flvors (often a result of the recipe formulation) etc. I’m not a huge fan of overly sweet pastry stouts (although I brew them myself), but done right they can be nice dessert drinks.
While most commercial brewers have a reasonable level of control over all the brewing parameters, faults will occur at times. There’s a saying in the brewing industry that goes like this: «There are two kinds of breweries; those who have had bacterial infections, and those who will». Being a brewer is in essence a glorified cleaning job. We strive to be as sanitary as possible. It is impossible to create a completely sterile environment for the beer, but we go to great lengths to ensure that the yeast will have no competing bacteria or wild yeasts to fight during fermentation.
So, the worst beers I have had? Most of them stuff made by novice homebrewers without enough knowledge of the process, and a few commercial products where the beer has developed infections after bottling. I have judged a lot of homebrew competitions, and I have tasted foul liquids so bad that the smell alone made me want to die. Especially since I knew I HAD TO taste it. In situations like that it is a great consolation knowing no pathogenic bacteria can survive in beer under normal circumstances... There are plenty of mediocre products out there as well, but I never agree with people saying things like «Bud tastes like foxes pissed in a can», or «Heineken tastes like soapy water». Personal preference is one thing, really BAD beer is something completely different.
It is far more easy to name favorites. But again, that’s just personal preference, and very often linked to the setting in which I drink them (or tasted them for the first time).