Almost every where is good, but my personal preferences are for:
1) Developing countries, because they are cheaper, there's less traffic and carrying bikes on public transport is often easier.
2) Ex-communist / soviet bloc countries because there's a history of collective land ownership so wild camping is very easy.
3) Muslim countries, e.g. Turkey, Iran and all the 'stans, because they are very hospitable.
4) Big mountains, e.g. the Himalayas, Andes, because that's what it's all about.
5) A reasonable network of secondary roads or cycle paths, so you are not tied to the main roads.
I tend to avoid:
1) Very developed countries, because there's too much traffic and they are expensive, so I'm not in a hurry to tour in the States..
2) Very flat regions (sorry Holland).
3) Countries with high population densities.