I follow Goodreads’ general scale, as I found it worked really well for me.

Here are the ratings:
Stars Description
It was not worth the paper (or digital file) it was written on.
I didn’t like it. Tough to read.
It was okay. But it wasn’t the best thing ever. Just a nice book.
I liked it. I enjoyed the book and I didn’t hate it or absolutely love it.
I really liked it! This book was so good and it was hard to put down. Totally enjoyable.
It was amazing. This book was unbelievably awesome and I don’t want to stop talking about it ever.