Sarcoidosis: This condition is known for causing an elevation in Angiotensin-Converting Enzyme (ACE) levels and can also lead to parotid swelling due to the formation of granulomas in the parotid glands. The combination of high ACE levels and parotid swelling makes sarcoidosis a strong candidate for the single most likely diagnosis.

Sjögren's Syndrome: An autoimmune disorder that can cause parotid swelling and may also be associated with elevated ACE levels in some cases. It primarily affects the exocrine glands, especially the salivary and lacrimal glands.
Lymphoma: Certain types of lymphoma can cause parotid swelling and may be associated with elevated ACE levels, although this is less common. Lymphoma should be considered due to its potential impact on multiple body systems.

Tuberculosis: Although less common, tuberculosis (TB) can cause parotid swelling and may be associated with elevated ACE levels. TB is a critical diagnosis not to miss due to its infectious nature and the need for prompt treatment to prevent spread and complications.
HIV/AIDS: Infection with HIV can lead to a variety of symptoms including parotid swelling (especially in children) and may be associated with changes in ACE levels. Given the significant implications of HIV/AIDS on health and the importance of early diagnosis for treatment, it is a diagnosis that should not be missed.

Heerfordt's Syndrome: A rare form of sarcoidosis that specifically affects the parotid gland, uvea, facial nerve, and sometimes the central nervous system. It's characterized by a combination of symptoms including parotid swelling and could be associated with high ACE levels.
Mikulicz Disease: A rare condition characterized by the enlargement of the salivary and lacrimal glands, which could potentially cause parotid swelling. While it's less likely to be directly associated with high ACE levels, its rarity and the specific symptom of parotid swelling warrant its consideration in a differential diagnosis.