Portugal,
Popularity measured by "percentage of population fishing for bass" is i believe still unknown, but it can also be measured by other indicators such as:
Competition Level:
For what i know Portugal, Spain and Italy are the only European countries to have official Bass Fishing National Championships. In Portugal and Spain there are "boated" and "from the shore" championships in Italy i don't know and in France the National Championship is not Bass specific as other species are also fished.
Italy, Spain and Portugal host important International Bass Fishing events.
The FIPS Bass Fishing World Championship held for the past 3 years has representatives from Portugal, Italy, Spain and France. Other countries like Russia (fishes in Cyprus), Mexico, USA and others have had 1 or 2 participations as well. Past winners were Portugal 2005, Italy 2006 and current title holders Spain 2007.
Press Coverage:
Italy is i believe the only country in Europe to have a "Bass fishing only" regularly published and publicly available magazine. Portugal, Spain and France have generic fishing publications.
Overall i believe Italy is the European country where the sport is more developed followed closely by Spain and Portugal.
P.S. - Contrary to what is thought there is no "Floridanus" fishery in Portugal or Spain at least not in public waters.