Flemish isn't really one language, but it rather refers to a variety of Dutch dialects (usually) intelligible with each other spoken in Belgium (and in northern France, but the language is dying out there). So someone from Antwerp, someone from Gent and someone from Leuven all speak a dialect of Dutch that you'd call Flemish, but each is different. West Flemish is often distinguished because it is WAY more different from standard Dutch than other Flemish dialects, to the point that it is barely intelligible with other dialects, and that some linguists think it should be considered a separate language.
It's a bit like Low German, or Swiss German. They still share a lot with standard German, but the differences are so big that it can be difficult (or even impossible) for someone who speaks the standard language to understand it, and they are therefore sometimes considered separate languages.
All Flemish people can speak standard Dutch nowadays, but most of them speak a dialect (or a mix of dialect and standard called "tussentaal", literally "language in between") at home or with friends.
DISCLAIMER: I'm a French speaker, but I'm interested in linguistics.
And as usual you're right. Brabants (C) is the dominant regiolect. I'd rather use this term since the dialects grew this close that some became indistinguishable. Since the main cities are all in the region - with the old name of - "Brabant" , its regiolect got to influence the use of language the most in Belgium. Nowadays people try to find a balance between AN and Tussentaal. Personally, I always speak "Verkavelingsvlaams" (another term that in my opinion describes Tussentaal better) .