Mild:
* Melancholy: A gentle, wistful sadness.
* Downcast: Depressed in spirits, lacking energy.
* Dejected: Disheartened, discouraged.
* Glum: Somber and moody.
* Dispirited: Lacking enthusiasm or courage.
* Mournful: Expressing sorrow or grief.
Moderate:
* Sorrowful: Filled with sadness, especially due to loss.
* Grieving: Deeply sorrowful, especially for the death of someone.
* Wistful: Longing for something lost or unattainable.
* Despondent: In low spirits from loss of hope or courage.
* Heartbroken: Overwhelmed with grief or sorrow.
Strong:
* Distressed: Deeply troubled or upset.
* Anguished: Experiencing great mental or physical suffering.
* Miserable: Extremely unhappy and uncomfortable.
* Desolate: Completely alone and without hope.
* Despairing: Losing all hope.
Formal:
* Pensive: Engaged in deep thought, often sad or melancholy.
* Woebegone: Looking or feeling sad and wretched.
* Lugubrious: Excessively mournful, especially in a gloomy way.
Informal:
* Blue: Feeling down or depressed.
* Down in the dumps: Feeling very unhappy.
* Gutted: Extremely disappointed or saddened.
The best synonym for "sad" will depend on the specific context and the emotion you want to express.