For me, the worst book is easily The Beginning, #54. That wretched book has haunted me. Seriously, every time I think about it... I also hated the casual way KA talked about Rachel: "Oh, yes, poor Rachel." ARGH! I understand the main message in the series was "war is hell" but the good guys lost so much...
Also, I hated The Return, #48. I felt like I was in a bad dream; the reader didn't know what was real and what wasn't. It was lousy. Plus, not knowing Rachel's decision was incredibly annoying.
I also disliked MM #4. It was too depressing and the cover (inside and out) made me want to vomit.
Actually, pretty much every book from #49 and up I despised. The new kids on the cover... knowing the series was coming to an end... those months reading crappy book after crappy book... that was a bad time.
Book #54 is the devil!!!