The advice I usually give to customers is:
* .org if you're a non-profit with international presence.
* .com is good for most international businesses.
* .eu can be used as an alternative to .com for businesses and organizations in the EU.
* Country-specific tld's should only be used for services aimed at those specific countries.
* Avoid .net unless you're a network operator (edit: the same applies to .aero unless you're doing aviation stuff).
Anything else can be considered to be of novelty value only. Do not base your business on one of the new gtld's, but feel free to use them for landing pages or specific campaigns. Skip the cool domains like .io. Exception: .nu, which has long been used as a secondary tld in Denmark and Sweden.