Pretty much any American macro lager, including Busch, Bud, Miller, Natural, etc. Unfortunately this stuff dominates the American beer market, giving the many finer brews also called 'beer' a bad name.
One easy way to judge whether a beer is bad or not is by the color of its can. the so called "silver and blues" are generally the bad beers. ie beast, natty, busch etc... This 'judging a book by its cover' works every time.
Two words: Genny Light. Perhaps a slight mix between gasoline and sink water, this is simply not worth the intoxication. Your tongue will thank you if you just chugged rubbing alcohol instead. This is also a mainstay at Lehman Hall...so bring your own beer- unless you ask for different.
Pabst Blue Ribbon, Rolling Rock, and Coors are three generally recognized exceptions, each having a devoted following of discerning tasters despite their budget pricing. Keystone Light also has a devoted fanbase.
- This is like the avid skier that regulars at Jiminy Peak. Sure it's close, convenient, and cheaper, but given the opportunity, he'll go for something better. The above beers simply are not brewed with as much care, nor are their ingredients high quality.
Don't be fooled into believing a beer is necessarily good if it is imported. Some of the overpriced, well-marketed "Euro" beers (e.g. Heineken or Stella Artois) can be just as bad as the American macros.
And in case you are pretentious classy enough/actually enjoy having a good beer, check out the Good Beer page for some fine suggestions.