Another Sherlock Holmes fan here
Sentimental reasons, my dad used to read them to me. My other childhood favourite which I keep re-reading is Alice in Wonderland, I still have the illustrated version my grandmother gave me.
Two non-kid books which hold a special place for me are Wuthering Heights (the first 'classic' I ever really enjoyed) and The Chrysalids (first sci-fi I ever enjoyed). I have read both of them multiple times.
Also, I am not sure this is a 'favourite' but I had to read Alice Munro's Lives of Girls and Women about 4 times in university. They like to pick a Canadian book for the first year course for the people who won't take English again, and then it came up again in Canadian Literature, Post-Colonial Literature and Women in Literature. It was a fine book, but I am not sure I would choose it four times if I didn't have to
The funny thing is, every lit major I have ever known had a book like this. My dad swears he had to read Great Expectations about 6 times.