Depends what you want out of the ride.
If you a fit and lean then the Alps or the Pyrenees would do.
If you are an unfit stoner then Amsterdam for a fortnight - that's a life time if your a hippy.
German Radweg - cycle paths are well sign posted and there's a great range of Bikeline books describing the routes.
Eurovelo Six is well sign posted and follows the Loire and the Danube from St Nazaire in Brittany to Constanta in Romania
Do you want to:
Enjoy scenry?
Enjoy museums?
Be unchallenged by major climbs?
Be challenged by major climbs?
Do large distances daily?
Do moderate distances daily?
Want to camp?
Want to use hotels/b&b/hostels
Fly to the start?
Catch a ferry to the start?
Catch a train to the start?
I've cycled:
From the Channel to the Med
From St Nazaire to Budapest in three instalments
And enjoyed nearly every kilometer.
Have you cycle toured before?
Research:
Eurovelo Six
Elbe Cycle Route
Rhine Cycle Route
Danube Cycle Route
A bit more information from you will generate more useful answers.