Highest of all my books is Way of Kings by Brandon Sanderson at 4.65 (others are higher but it's just people rating unreleased books 5 out of hype). I haven't read this yet but it's very high priority for me, can't wait!

Highest I've read is Harry Potter and the Deathly Hallows, at 4.62. It's nowhere near the best book I've read and IMO it's not even the best Harry Potter book but I don't really care about HP anymore so it's whatever.

Lowest overall and that I've read is Stjernetrekker by Harald Rosenlow Eeg at 3.00 (based on just 8 ratings). It's a norwegian book and a spiritual sequel to one of my absolute favourites as a teen, so I think very highly of it but I haven't read it in over ten years so I can't say if the low rating is deserved or not...