According to Granqvist and Svec, the best microphone for recording all natural speech is omnidirectional: http://www.speech.kth.se/~svante/Granqvist_Svec_PEVoC6_Microphones.ppt
Article Recommended Protocols for Instrumental Assessment of Voice: ...
Patel, R. R., Awan, S. N., Barkmeier-Kraemer, J., Courey, M., Deliyski, D., Eadie, T., ... & Hillman, R. (2018). Recommended protocols for instrumental assessment of voice: American Speech-Language-Hearing Association expert panel to develop a protocol for instrumental assessment of vocal function. American Journal of Speech-Language Pathology, 27(3), 887-905.