Strongest:
* Abhorrent: evokes a feeling of intense disgust and revulsion.
* Appalling: shocking and horrifying.
* Dreadful: extremely unpleasant and frightening.
* Heinous: extremely wicked or evil.
* Monstrous: extremely ugly, cruel, or unnatural.
More nuanced:
* Abysmal: extremely bad or dreadful.
* Deplorable: deserving strong condemnation.
* Execrable: extremely bad or unpleasant.
* Repulsive: causing disgust or aversion.
* Despicable: contemptible and deserving of scorn.
Informal:
* Awful: very bad or unpleasant.
* Terrible: very bad or unpleasant.
* Horrific: causing horror or fear.
* Disgusting: causing revulsion or disgust.
The best word to use will depend on the context and the specific meaning you want to convey.