I'm reading the kindle edition of this book. I doubt there is anyone who hasn't heard of Mr. Holmes yet but it's not wrong to repeat - it's one of the greatest books ever.
There are a lot to love but here are the top three:
i) Hints - there are always enough hints to track the progress but never enough to reveal the suspense. Conan Doyle balances this really well. That's why Sherlock Holmes remains my all time favorite
ii) Cleverness - Seems like the author has a never ending pool of ideas. Story after story there is something or the other that makes you say wow.
iii) English - It's fascinating to see the choice of words and phrases of (not so) old English.