The superlative of fashionable is "most fashionable."
most fashionable
I am fashionable.
yes very fashionable.
That is a very fashionable hat!
They're not fashionable now.
Fashionable Lectures was created in 1872.
Fashionable Nonsense was created in 1997.
Comparative: more fashionable Superlative: most fashionable
Jessica Simpson's shoes are quite fashionable.
the most fashionable company is totally north face
The Dictionary of Fashionable Nonsense was created in 2006.