What's the best medicine for a lip blister?

Depends on cause: Lips will blister due to excessive sun exposure, viral or bacterial infection or lip biting or trauma. I hesitate to be more specific without know the diagnosis because treatment is different for each.

Lip blisters: I agree with dr. Alder that treatment depends on the diagnosis. If it's traumatic, orabase to protect it or carmex or aquaphor will make it more comfortable while it heals. If it's viral, like herpes, a topical antiviral like abreva or lipactin will shorten the duration. If it's fungal, an antifungal like Nystatin would be used. If it's bacterial, a topical antibiotic ointment such as bacitracin..