There is no single "best mic" because it depends what you are recording.

If you are doing dialog for a movie, a wireless lavaliere or two or more – or a shortgun mic. If you are recording music, stereo separation is nice, so a stereo mic. If you are recording loud audio, none. The Zi8 has no manual audio control and the auto-mic gain will be over-driven resulting in muddy or audio with lots of static (peaking). The best mic in the world cannot resolve this without manual audio control.

Since the Zi8 has no useful accessory shoe to mount a mic, add an accessory shoe by using a camera bracket. At amazon, do a search using "bower camera bracket". That angled bracket has two accessory shoes – one for a mic and one for a video light.

Check the Audio Technica PRO24 CM or NRG Reseach SA-800. Both are low-end stereo mics that work well.