...which is difficult to say, because all her works are classic, no exceptions.
1) The writing style. When you pick up a book by Jones, you can tell without needing to glance at the author's name. Witty is an understatement. All her books have subtle and not-so-subtle jokes woven into every paragraph, while at the same time writing books that are sensitive and dark.
2) The characters. Jones' most memorable characters are those like Howl from 'Howl's Moving Castle' and Jonathan from this book (Time City). Both are arrogant and smart, and abitious to boot. Jonathan, however, is not the foppish pretty-boy that Howl is. He is very much like that smart kid that is positively certain that he is the smartest, and may well be. He, like Howl, also has very real faults, but in a different sense. While Howl is liable to be messy and spend hours in the bathroom, Jonathan does things much more recognizable: chew on his hair (which may not be recognizable for boys necessarily, but it still hits home with a lot of readers), doubt people, and jump headfirst into things without thinking.
Vivian, the female protagonist, is also imperfect, but also acts as an anchor for Jonathan and Sam (Jonathan's younger cousin)'s outlandish ways. Just like us, she blushes when Jonathan sees her Liberty bodice, she itches in wool sweaters, and she sweats and feels dirty and hot and uncomfortable. She is like the anti-Jonathan, per se. While he is overconfident, she is somewhat self-concious and uncertain. The whole thing balances the book greatly, but still manages to be quite madcap in the same instant.
But it takes all three heroes/heroines to come out victorious over the rolicking, complex plot and all the stakes Jones sets up for them. But don't worry, they're up to it-- so I'd definitly recommend this book, which could most certainly be called a modern classic, to anyone who loves to devour a book with rich, realistic characters (despite the fact that some of them aren't entirely human--can you say 'android'?) a wonderfully complicated storyline, and general spunk and ingenuity.