Cover your bases:
G.723.1 most widely used for PCs and holds a MOS score of ~3.3
G.729 most widely used for compressed voice between gws with a MOS of 3.9
G.711 MOS 4.1 which is considered toll quality.
G.726 good to support but has no real benefit as the ADPCM algorithm is not as robust as the CELP flavour.
If I was building, G.711 and G.729 would be a must at release. Might as well go with G.729a as you can squeeze more calls out of the DSPs due to the complexity of the codec.
G.729 maxes the number of RTP sessions not SIP sessions.