So many! But I think the one book that I have read over and over again, to the point where my copy is truly falling apart, is Black Beauty by Anna Sewell.
I have spoken before about how this book likely contributed to me becoming the animal lover that I am, but that is why it is my favourite book of all time. Followed very closely by my other favourite book, The Strange case of Dr Jekyll and Mr Hyde by Robert Louis Stevenson.
Yes, I appreciate that those two books are very different genres.