My personal ranking and some wild guesses:
1. Hamilton - future multi-champion, as long as McLaren stay fast
2. Alonso - current multi-champion, might win again but probably not with Renault
3. Räikkönen - current champion, should win one more championship with Ferrari in 2008 or 2009 before retiring
4. Rosberg - surely a future champion
5. Vettel - possible future champion, amazing talent, relaxed and intelligent
6. Kovalainen - possible future champion, 2008 is make-or-break year
7. Kubica - possible future champion, but will he ever get a good enough car?
8. Button - one of the best out there but always in the wrong team, won't be champion
9. Massa - in the right team, but still won't be champion
10. Sutil - still a big unknown, but the boy seems FAST