I can't decide on an overall favorite book or series, so I'm going to cheat and list a few.
To Kill a Mockingbird by Harper Lee
The Price of Salt by Patricia Highsmith
The Valley of Fear by Sir Arthur Conan Doyle (or any of the Sherlock Holmes novels in general, but I think I rather enjoyed Valley of Fear quite a lot, even more so than the fan favorite Hound of the Baskervilles)
Jurassic Park by Michael Crichton
Frankenstein; or, The Modern Prometheus by Mary Shelley
Black Beauty by Anna Sewell
Never Let Me Go by Kazuo Ishiguro
I'd probably be able to list more if I had a list of every single book I've ever read at my disposal. Because I know there's at least one that should be here, but absolutely isn't.
Harry Potter serie is my forever favorite. I enjoyed every book as a child and I still enjoy the universe and the world the author created! It's just wonderful. I don't read them anymore (school asks me other things and other books) but I've read each of them at least three or four times!
The Little Prince could be my favorite book ever. I enjoy reread it again and again. The Little Prince and his friends are just lovely and it's a beautiful story. Please, just read it.
I loved The Song of Achilles so much! Didn't expect a gay romance but enjoyed it. So cute and so sad, you know the end even at the beginning...
Fun fact: I don't own a single book that isn't in a series. I either own the full series or at least one book that's in it. I do want to read Song of Achilles though. It's based on mythology and there's gayness involved. The Percy Jackson series also ticks off those boxes now that I think of it, so I should read that too.
My favourite series are The Hunger Games and Harry Potter. I haven't even read all four Hunger Games books, yet here were are. My favourite Hunger Games book so far is Catching Fire, and my favourite Harry Potter book is Half-Blood Prince, with Prisoner of Azkaban coming in a very close second.
Wolves of the Beyond by Kathryn Lasky for me. I haven't read her other series (which is Guardians of Ga'Hoole, blame Wings of Fire for that), but i may get around to it once I get the entire Wolves series (I have Watch Wolf and Lone Wolf, which are books 3 and 1, respectively, and I am missing Shadow Wolf, Star Wolf, Spirit Wolf and Frost Wolf, which are books 2, 4, 5 and 6).