I mean, you should probably
start off by mentioning Freddie Mercury, Whitney Houston, Mariah Carey, Celine Dion and Michael Jackson if you're talking about impressive singing voices in pop music.
But I assume you're also interested in distinctive or interesting voices.
Anyway, Anohni (previously known as Antony Hegarty, of Antony and the Johnsons, before transitioning) has an interesting voice. She collaborated with Björk on a few songs, including The Dull Flame of Desire which showcases her distinctive voice pretty well.
Oh yeah, and Björk is pretty cool too.
The song Blind with Hercules & Love Affair is a bit more commercially accessible though: