Here is mine (for fiction):
1 star - truly terrible, little or no merits. not even so bad its good kind of bad. Will most likely drop the book. Whether we should even rate books we haven’t finished is another discussion. However, I am of the believe if a book has nothing going for it even though you’ve read 100 pages, it’s probably a 1 star.
2 star - it was ok. Enjoyed bits here and there. It did not feel like a waste of time, but would not read again.
3 star - this one is hard to define. it was better than ok, but not great. would probably not reread, but fairly enjoyed.
4 star - very enjoyable, would not mind rereading. maybe there were a few flaws here and there, but nothing detrimental.
5 star - this book made me think about it for days, weeks, maybe even months. I care about the characters like they’re my own family. I would want to reread it again and again. Although, there are some books I would not want to reread, maybe because it dealt with dark or violent topics, but I would still rate 5 star because of the impact it had on me. Did the book change me, did it make me want to reread it, those are pretty good criteria for me to see if the book deserves 5 stars.
Note, I have never really rated books until recently. I realize it helps me reflect on how I feel about the book, and helps me get closure and move on to the next book without any unnecessary lingering attachment. Unless its a 5 star of course, then I’ll think about it forever, haha. I don’t even have a goodreads or do reviews or any of that jazz, just thought it would be fun to discuss.
I like to think of it as, if I was forced at gunpoint to reread this book, how angry would I be that I have to reread the book or I get killed?
5 - easy. Liked the book, easy reread. Having my life threatened is a great excuse to reread it.
4 - Still okay. It might take a while, but fine.
And of course 1 is essentially “I might actually prefer the bullet to rereading this book.”
The less gruesome scale is
5 - Great book. No notes.
4 - Good book. Some notes.
3 - Okay book. Several notes.
2 - Bad book. Many notes. Probably did not finish it.
1 - Awful book. Tons of notes. Probably did not finish. Would recommend this book for crafting purposes only.
Offhand I can’t think of anything I’ve read I would give 1 star. Even stuff I didn’t finish I’ve given 2 stars.