Hi,
You can get a rough idea of FR's largest markets by looking at the departing seat capacity graphs at the bottom of this page:
ELFAA - The European Low Fares Airline Association
Note that the largest markets may not necessarily be the 'best' markets, depending on how you choose to define 'best'.
According to the most recent (Oct 2009) points on the graph, FR's largest markets by monthly departing seat capacity are:
1. UK - roughly 1.7M departing seats per month (and declining)
2. Spain - roughly 1.3M (massive growth)
3. Italy - roughly 1.25M (massive growth)
4. Ireland - roughly 0.7M (declined to below 2007 levels)
5. Germany - roughly 0.6M (strong growth)
Domestic capacity in Italy appears to have grown rapidly to over 0.45M departing seats per month. FR have been developing a strong position in the Italian domestic market. Is it really worth jeopardising this for the sake of a fishing licence?