Tristessa, Big Sur, On the Road or Desolation Angels
Those are my favourites.
Tristessa because it was oddly funny.
On the Road since it was the first one I read. (Actually Ferlinghetti's Coney Island of the mind was, but it didn't know who he was).
Big Sur because there was a sadness that existed throughout the book.
And Desolation Angels, just because.