Strongest:
* Abhorrent: Evoking intense aversion or loathing.
* Appalling: Shocking and horrifying.
* Repulsive: Causing a strong feeling of revulsion.
* Loathsome: Extremely unpleasant and causing disgust.
* Abominable: Morally very bad; extremely unpleasant.
* Deplorable: Deserving strong condemnation; utterly unacceptable.
Moderate:
* Offensive: Causing displeasure or disgust.
* Nauseating: Causing a feeling of sickness or disgust.
* Repugnant: Strongly distasteful; highly objectionable.
* Unpalatable: Unpleasant to taste or experience.
* Revolting: Causing a feeling of disgust or revulsion.
More specific:
* Squalid: Dirty and unpleasant, especially through neglect.
* Putrid: Having a foul smell; decayed and rotten.
* Rancid: Having a strong, unpleasant smell or taste, especially due to decomposition.
* Grotesque: Disturbingly odd or ugly.
* Ghastly: Horrifying or frightening.
Informal:
* Yucky: Disgusting or unpleasant.
* Gross: Extremely unpleasant or distasteful.
* Blech: Used to express disgust.
* Ugh: Used to express disgust.
The best word to use will depend on the specific context and the level of intensity you want to convey.