For all intensive purposes, G711 gives the best quality with 10 ms packetization period is the best.
G711 samples a 0Hz to 4kHz frequency range. If you dont go with a standard codec you can probably get better quality out of a platform. Traditionally proprietary codecs work great, but then it wont work with other folks voip endpoints if they dont know how to code or decode those codecs.
There are some wideband codecs out there that will sample a wider range but I havent seen any implementations of those especially since they generally use higher DSP MIPS. Those will give better than toll quality voice.
Lastly, I have been hearing increasing interest in the GIPS(Global IP Sound) codecs. It seems as if they have enhanced many traditional codecs to be used for Voice over IP. If you know, or find a vendor that uses their codec they may have a lower bit rate codec with better quality voice under adverse conditions. This still presents the same issues as above…interoperability