Does Manga count? If so, I'd say Mahou Sensei Negima! Due to the ending...WHO DOES HE LIKE DAMN IT!^This. Seriously.
Claymore and EvangelionThe thing about Claymore (as well as Gantz and so many others) is that the original work was cut in half to fit the format, for budget issues, censorship, or whatever reason. I personally hate when ongoing works make it to anime, because I know for sure that if it isn't a massively mainstream shounen series, they will make up a nonsense ending or a weird clifhanger to a second season that never comes.