I love unique voices. From Michael Stipes mumbling slight twang to the nasally whines of Gordon Gano from the Femmes, Jeff Mangum from Neutral Milk, and Colin Meloy of Decemberists. The voice is the glue that makes or breaks bands for me.
But if I can indulge and push the envelope of what is "alternative" a smidge, one of my very favorite voices of all time loves to sing of love and fantastical environments, who feels almost genderless and ethereal, and whose music rarely dipped into the mainstream (just once or twice). Is prog alternative? Probably not, but it ain't mainstream.
So many songs to choose from that I love so much, but I'll narrow it to 3.
A beautiful song with one of the best bass drops in history from what's considered one of their best albums Close To The Edge: "And You And I" - https://youtu.be/IwpB6b6mhUM?si=rUuD9jr7hr7e7ovu
A massive sprawling epic overshadowed by the song "Close To The Edge" called "Awaken" - https://youtu.be/59feSG8U5EQ?si=snZJT9xbatKJBt32
And finally, a much later epic from Keystudio, a song called "Mind Drive" which dips toes into almost-jazz fusion jam - https://youtu.be/zvBknHFa8PQ?si=ocB4VViT1RH71sjS