The worst ending I've read recently was the conclusion to David Weber's
Out of the Dark. It's your basic alien-invasion story, where the humans hold their own through ingenuity and refusal to give up, but are increasingly hard pressed. You wonder what they are going to pull out of their hats to defeat the alien oppressors, and then...well, they're rescued. I can't even bring myself to mention by whom.
Semi-spoiler follows: