Adding an external microphone is the single most significant way to improve the quality of the sound your phone captures. So, if you’re serious about giving your videos and voice recordings top-level audio, you’ll need to invest in the best mic for recording.
If you primarily plan to save the sound of a person speaking, look for a directional microphone, sometimes called a shotgun mic. You can literally point this type of recorder at the audio source to pick out one pocket of sound. To record a broader area, perhaps if you plan to film a movie or record crowd sounds, then the directional mic isn’t quite as necessary.
Hire this guy to walk with you. That's what the professionals do.