Below are five favourites of mine. I could name many more.
1). Emma - Jane Austen
2). Little Women - Louisa May Alcott. (In my opinion the best American novel).
3). The Lantern Bearers - Rosemary Sutcliff
4). Arthur High King Of The Britons - Michael Morpurgo
5). Changeover - Margret Mahy
Every time I even think of the first scene in Pride & Prejudice between Mr & Mrs Bennett I have to pick it up and reread it. In fact...
There aren't many I reread, but if you read A Little Princess in your formative years, definitely try it again. I have no idea why, but that was a completely different book a decade after my first read.
How difficult is that question? I love all the Jane Austen books and sometimes read them again when I need cheering up. I loved Anne of Green Gables when it was first introduced to me when I was 11 years old. I often read it again. I have just read The Road by Carson Cormack again and was glad I did. I think I got more out of it this time. Oh and there are a hundred others...But so much new stuff out there as well.
I would say ~ The Sherlock Series especially Hound of the Baskervilles....