Bell's Palsy: This is the most likely diagnosis given the presentation of unilateral facial weakness, which is a hallmark of Bell's palsy. It is an idiopathic condition that affects the facial nerve, leading to sudden onset of facial paralysis or weakness.

Stroke (Ischemic or Hemorrhagic): While less common than Bell's palsy for isolated facial weakness, a stroke can cause similar symptoms, especially if it involves the brainstem or the area responsible for facial nerve function. The key differentiator would be the presence of other neurological deficits.
Lyme Disease: Neuroborreliosis, caused by Lyme disease, can lead to facial nerve palsy, among other neurological manifestations. This diagnosis would be considered, especially in endemic areas or with a history of tick exposure.
Ramsay Hunt Syndrome: This condition, caused by varicella-zoster virus reactivation, can lead to facial paralysis along with other symptoms like ear pain and vesicles in the ear. It's a less common cause but should be considered, especially with additional symptoms.

Central Nervous System (CNS) Tumors: Although rare, tumors affecting the brainstem or the cerebellopontine angle (like acoustic neuromas) can cause facial weakness. Missing this diagnosis could have severe implications.
Multiple Sclerosis: This demyelinating disease can present with a wide range of neurological symptoms, including facial weakness. Early diagnosis is crucial for management.
Cerebral Venous Thrombosis: This condition can mimic stroke and cause facial weakness among other symptoms. It's particularly important to consider in patients with risk factors for thrombosis.

Sarcoidosis: This autoimmune disease can affect any part of the body, including the nervous system, leading to facial nerve palsy. It's a rare cause but should be considered in patients with other systemic symptoms.
Guillain-Barré Syndrome: An autoimmune disorder that can cause ascending paralysis, which might initially present with facial weakness. It's rare but critical to diagnose due to its potential for rapid progression and respiratory compromise.
Tolosa-Hunt Syndrome: A rare condition characterized by painful ophthalmoplegia, which can also involve the facial nerve, causing weakness. It's due to nonspecific inflammation in the cavernous sinus or superior orbital fissure.