What is a dolphins best friend?

Dogs.

Answer:

No. While there are unusual, well-supported cases of porpoises assisting drowning men, humans do not by any means share a symbiOSis with porpoises, as we do with domesticated dogs.

Answer:

Recent studies have shown that dolphins are colblooded (in the sense of merciless and like to do it) killers. (See Link). Not the kind of large toothy animals that you'd lie to think were friends until its too late. The smile on their faces is an accident of evolution, not an indication of a pleasant nature.