What is the most effective treatment for plantar warts?
Unfortunately, there is no "most effective" treatment when it comes to any kind of wart. Treatment is based on the premise that creating an inflammatory response in the wart infected tissue will result in your body's immune system killing the virus. There are many ways to inflame tissue so all these are tried with equal success (or lack there of). Freezing with liquid nitrogen (painful on the bottom of the foot with the potential to cause scarring), topical applications of mild acid, occlusive dressings (like duct tape) are all potential treatments. They can sometimes help with the warts disappearing more quickly and other times, seem to actually stimulate the wart growth.
Many care providers advise simply living with the warts, keeping the callous formation over them sanded down with pumice stones. Eventually, they will go away on their own.
If you do try treatments, be persistent and regular in the application of the treatment until the warts resolve. That can require frequent visits to clinic if doing liquid nitrogen applications.