You know, i was thinking of putting Mylene on there, but like Mao she doesn't really qualify, she doesn't really sing her own songs and really only provides backing to basara, so i guess that makes it tougher, but hey what can you do. She has talent no doubt, but being in the same band as basara really has put a cap on her being a proper songstress.
For me the decision is really hard, song wise having the best songs, i'd say probably sheryl or ranka, who have had the best choices of songs as of yet. However as a character and seeing through battles like a proper songstress, you would have to give props to minmay and basara too, probably more to basara since he is in the thick of it aswell whereas the rest are singing from a macross class ship or larger.
Nobody has mentioned sara or ishtar yet. Sara having survived a forest nuke and suceeded in waking the protoculture, which no one else has ever done with their song, but i guess others have summoned vajra and protodevlins instead.... but come on the protoculture was pretty epic. Ishtar on the other hand is a songstress for the marduk, one of many apparently, but the only one to perform to the best of her abilities ontop of an exploding macross in the inevitable face of doom, give her some props for that.