I must be the only one to think Death Note ended too late. It should have stopped at the finale of the Light v L arc, and not just because L's replacements were...lacklustre at best. Has anyone seen the movie? Now THAT would have been an ending for the manga. If you haven't seen it, you're in for a twist.
Speaking of anime I loved Elfen Lied, but since I've read past the part where the show ended I know what really happened to Lucy, and I know what would have happened next, and I didn't much like the rest of it. Besides, the series ended on a sad note, and isn't that the point of Elfen Lied? It's brilliant, but that stuffs depressing.
And for my own finale; a series that should NEVER have even begun! Life on Mars (US version). I mean really, the whole 1973 revival thing was due to the UK's Maggie Thatcher against the Unions era of British history... why did America see fit to copy it? Makes no sense.