Here’s a post about unsatisfying endings at Book View Cafe:
Lovely characters, amazing setup, incredible building of the plot . . . It was all going so well. I so trusted the author to end it well. I was fairly wriggling in my chair to find out how she’d pull it together. And then—
What did she do? She cliffhangered the ending into a part two book.
Oh, yeah. We’ve all been there. In fact we could have a WORST OFFENDER EVER contest for Surprise! Cliffhanger! endings
Personally, I don’t think any book ever is going to beat Pegasus by Robin McKinley.