Top leagues would have to be:
Spain
England
Italy
with:
Germany
France
a notch below
The nice concept of soccer is they have the domestic leagues, then qualifiers for the next year's Champions League, even have an NIT-like UEFA Cup where the winner is in the Champion's League.
Most leagues will have a League Cup tournament as well which features every team all the way down to the local pub teams and can make for some fascinating underdog stories.
They also usually throw in another tournament.
I follow the English Premiership because I frequent an English bar and run into visitors from every part of the UK.
There are certainly parts of soccer that are despisable, but there are a few things that are hands down superior to American sports:
1) The concept of relegation and promotion (where the bottom teams are sent to a lower, less lucrative league and the top teams are promoted)
2) The all-encompassing League Cups where every team in the country starts on relatively equal footing for glory.
3) The rewarding of a great season by going to "Europe" as they call it(Champion's League play)
4) The local "derbys" or grudge matches which pit rival vs. rival, town vs. neighboring town, religion vs. religion.
These are just a few ideas off of the top of my head.
If I were you I'd find a local watering hole that shows the games of a particular league and study the teams and talk to the fans. Football fans can't wait to talk about who they support.