Final answer:
Levodopa-carbidopa is the best drug for Parkinson's disease but may eventually stop working.
Step-by-step explanation:
The best drug for Parkinson's disease that eventually stops working is Levodopa-carbidopa. Levodopa-carbidopa is a commonly prescribed drug for Parkinson's as it converts into dopamine by neurons in the brain, increasing dopamine neurotransmission and compensating for the loss of dopaminergic neurons in the substantia nigra. However, over time, patients may become less responsive to Levodopa-carbidopa treatment.