It's impossible to choose just one, so here's three of the worst films I've ever seen.
Manos: The Hands of Fate. Literally translated as Hands: The Hands of Fate, Manos is an ultra-low budget and very obscure film brought to light by the fine folks at Mystery Science Theater 3000. The plot, if it can be called one, is about a cult leader, his satyr-like servant, and a family that get lost on a desert road.
Van Helsing (2004) - Starring Hugh Jackman as the legendary vampire hunter Van Helsing this film combines multiple famous movie monsters into a horrible mess of a plot. Abbot and Costello meeting the Universal monsters manages to be more frightening and less damaging to the legacy of monster movies than this movie. Van Helsing remains one of the few films so bad that I never actually finished the film, I turned off my TV, set down my remote and walked away.
Twilight (or any of its sequels) - Boring, uninteresting, bad scripts, bland characters with no chemistry and makes a mockery out of vampire/werewolf folklore. Even the action sequences aren't well choreographed enough to raise your pulse.