Hm as a ELF, siwonest and petal I don't really have as top vocalists from K.R.Y. all set up.
For me, the best singer is :
1. Kyuhyun, his voice is sweet and it's really to give shills and makes me cry (proof of that is "Late Autumn", in "Coagulation", "7 years of love", "Blue Tomorrow" with a sweet voice,...) but then, there it is, it doesn't came Wookie or Yesung. The one I think it sings better
2. is Siwon - ok there it is, I'm suspect but just hear, for e.g. "Worthless" and his voice in "Someday" omo rly, people are to much attached to the idea of KRY, but sometimes... I dunno if it would be better to analyze better this,
3. Yesung, (it's like Kyu but in a different way, he got a sweet and calm voice and awesome balance, "A Man" (Immortal Song) I think is the best to see this, it's really amazing)
4. Zhou Mi, (he know where to low and up his voice in perfect style, same to Donghae)
5. Hangeng (just wow "Clown Mask", do I have to tell more...),
6. Donghae, ("Gulliver", ...)
7. Eunhyuk, (funny voice like Teuk but is different, and ok that he's better at rap, but hum... "Gulliver" too and "Bonamana",...)
8. Ryeowook, (i dunno and please don't be mean but i like his voice but is too high in some musics, like... "Blue Tomorrow" is ok, smooth but still him, but for e.g. "Opera","U",... he exaggerates)
9. Heechul (people say awful things but "Close ur mouth" and Breakups are so typical of me" can shut some bad talk),
10. Kangin, (maybe i'm really being unfair with him, 'cause he haves a 'unique' type of voice)
11. Henry, (unfair too... but better playing violin)
12. Leeteuk, ('funny' voice, i tell this because of SPY but is better mc)
13. Sungmin, (I still have doubt about his cappacity, maybe I'm being mean too xD)
14. Shindong, (better in rap)
15. Kibum (not 'cause is absent, but even in the songs I heard... well...)
but is impossible to say that they sing bad, 'cause none of them don't. I don't say either that Shindong is bad at singing, but he's the 2nd best on rap.
So... I disagree with this http://www.youtube.com/watch?v=vO0GHoGj2oM&feature=related and the songs were not the best to some of them.