Voice, lyrics, and intangibles all factor into making a great lead singer. Some on this list don't have a "top ten voice" alone, but make up for it in the other areas.
1- Maynard James Keenan (best voice in rock, best lyrics in any genre. Best ever IMO)
2- Serj Tankian (Incredible range and very distinguishing voice with great content)
3- Brandon Boyd
4- Corey Taylor
5- Eddie Vedder
6- Chino Moreno (originality points and very artistic lyricist)
7- David Draiman (just love his voice, and his energy live is amazing)
8- Amy Lee (gorgeous, haunting voice)
9- Roger Waters
10- Chris Volz
I want to put Zack de la Rocha and Trent Reznor, but neither has a great signing voice. Great for their genre, and they have all of the intangibles and content. I just don't think what they do vocally is elite. The people listed above are hard to imitate, they have rare vocal talent.