General:
* As ugly as sin
* As bad as a bad dream
* As dark as a bottomless pit
* As cold as a grave
* As sharp as a knife
* As bitter as gall
* As heavy as a lead weight
* As empty as a void
* As rough as sandpaper
* As unpleasant as a toothache
Specific:
* As loud and jarring as a car alarm (for a horrible sound)
* As dry and tasteless as sawdust (for a horrible meal)
* As confusing as a tangled mess of string (for a horrible situation)
* As suffocating as a thick fog (for a horrible atmosphere)
* As painful as a bee sting (for a horrible experience)
The best simile for you will depend on the specific context and what you want to emphasize.