Compiled from what I've heard, read and experienced first-hand, my top 10 countries for cycling would probably be:

1. Netherlands
2. Finland (some people put Finland first, because of the much richer and vaster natural scenery)
3. Iceland
4. Sweden
5. Denmark
6. Norway
7. Austria
8. Switzerland
9. Belgium
10. Germany

Eccept for Iceland, I have been in all of those countries, at least briefly.