The best file format to record voice is:
AAC - if you want a better audio quality.
AMR - if you want a smaller file (in kilobytes, megabytes,...)
What is AAC?
Advanced Audio Coding (AAC) is an audio compression standard that has evolved from MPEG2 - Layer 3 (MP3). It was developed with the audio fidelity (quality) in mind. That is why it is currently used, for example, in digital broadcasting systems.
What is AMR?
GSM/AMR (Adaptive Multi-Rate) is an audio standard widely used, for example, to transmit voice over the cellular networks. It was based on the Algebraic CELP (ACELP) algorithm and operates at bit rates ranging from 4.75 Kbps to 12.2 Kbps. AMR was developed thinking, firstly, in reducing bandwidth but, trying to preserve some speech quality under severe transmission conditions. That is why it is currently used, for example, to transmit voice over 2G, 3G and other wireless networks.
If you want to record only voice, we would recommend AMR as its file size is many times smaller than the AAC file size.