@Muganda - I concur...however, when I look at what I consider my top 5 female vocalist, my criteria varies broadly such that even for just ONE song, a place is earned, or for consistency, or impact....who I think are my top 5 vocalists (and why)...
1 - Whitney Houston (consistent, powerful, emotional, moving....songs where her vocal prowess are most noticeable: "On my own" and the US National Anthem (there is a lot of debate about which female singer really infuses emotion and pride in the "Star Spangled Banner" - I think she does and have compared performances by Mariah Carey (useless!), Jordin Sparks, etc
2 - Sara McLachlan (bite me...I edged out Celine Dion for her...but I think she sounds like an Angel)
3- Lauryn Hill - (she makes it into my top 5 for just ONE song ("His eye is on the sparrow" with CeCe Winans) I wish she did more gospel stuff...and I also wish she never belonged to the Fugees - wasted time IMHO. Her songs in Sister Act really showed her prowess....
4 - Brandy (mainly for "Come a little bit closer" and the link I gave above)
5 - I think this slot should be shared between ...Alicia Keys, Jordin Sparks, Mary J Blige and Faith Evans....
BTW, I'd also put our own Susanna Owiyo in top ten for vocals.
For male vocalists, not in any particualr order - James Ingram, Brian McKnicht (but just for ONE song - "One last cry"), Lionel Richie, Elton John (bite me!)....