“Mahal ko ang aking best friend” would be the colloquial way to say it. “Mahal ko ang pinakamatalik kong kaibigan” would be the all-Tagalog way. - (Pinaka)matalik na kaibigan = (the most) close friend. Either way would exactly mean, and would have the same nuance as saying, “I love my best friend” in English.

Hi George The verb for Love is 'Mahal' but i don't think it gets used for friendly relations. I would say Gustong gusto ko ang pinakabuting kaibigan ko or Gustong gusto ko ang close na kaibigan ko