Oh - but those big blue eyes, filling with pain and horror as Angel achieves true happiness mid-point, turns into Angelus, and bites him...

Of course this might be before the quest - Angelus gets the ring, in more ways than one... Or possibly he turns Frodo and, as Angelus rampages around the Shire, the Fellowship numbers slowly dwindle as Frodo drains them one by one...

But really I'd have to push for Angel/Arwen - both of them sitting around, never being truly happy, for eternity - unless one of the twins stakes him of course...