What is the most effective treatment to get rid of plantar warts?

Several treatments. You can try over the counter wart removers, but be careful: i've seen them do more harm than good. Other options your doctor might offer include cryotherapy (freezing the wart), electrocautery (burning the wart), laser treatment, or several different topical chemical treatments. If the treatment is typically uncomfortable, your doctor can numb the area before proceeding.
Surgical excision. Although far from the only treatment, seems to work best in my experience.