G729a is the best codec to use to get quality and minimal bandwidth usage...but it requires licenses for each channel. $10/channel.
GSM codec is what i'm using, and it's quality is "fair"...bandwidth usage is very low...but it requires a little bit more CPU processing to transcode. No license required.
There's also Speex, which I haven't had a lot of experience with myself...but is also low bandwidth and no license required.
Then there's iLBC, which i've never used either.
.....being that i'm using GSM with no issues, I guess I would recommend that one first.