Tricky.
My first Jilly (you never forget your first Jilly
) was "Emily". I bought it at a garage sale done by friends of my parents.
I was 12 and it was the rudest thing I'd ever read!
(I also thought I'd seen a television programme based on it. If I did, I've no idea exactly when, and have never been able to Google it)
Iirc, I went through the next of the "name" books: Imogen, "Prudence", "Bella", "Harriet", "Octavia" and (much later) "Lisa & Co". Like a PP I had the ones with Jilly on the cover, got up to look like the heroine.
The latter was a book of short stories.
My local library had a very good selection of her non-fiction works.
Not, by any means, an exhaustive list but I recall: How to Survive from 9 to 5; Jolly Super; Jolly Super Too & also Jolly Superlative. (Sp? "Sup-ur -lative"?)
I then found a poetry anthology "The English in Love" and have got away with seeming well read based on my knowledge of that for years.
She is very sound on literature and doesn't wear research lightly, so you can pick up accurate trivia about eg horses and music based on her books.
Carried on through "Class" and "How to Survive Christmas" and a few others.
Then I bought "Riders" which had just been published....
Gosh! 

I bought it as a holiday book.
I couldn't tell you anything about the holiday
but I bet I'd pass an exam on the book.
Any road up...
I'd suggest reading "Riders" first, but buy "Rivals" at the same time as you buy "Riders", because you'll want to keep going.
Then the rest of the Rutshire chronicles (although her writing deteriorates with time and some of the themes are not in tune with social mores of today).
Then maybe the name books.
But her non-fiction stuff is also good.
And give "Class" a whirl, if only to see how things have changed.
Or not...