I'm going with Dak and Aldrea. Their relationship was just so much more epic than any of the others. Even Elfangor and Loren...the turmoil in their relationship added up to a week. One week! Aldrea and Dak had to survive a war together, and not just any war, a war that they were destined to lose. They also didn't have a perfect relationship. Aldrea used Dak to achieve her goals. She pulled the whole arrogant-Andalite card. She wasn't good for him immediately following her family's death. Dak was influenced by Aldrea to turn his kind into killers after centuries of peace.
But, despite all of that, despite all of the turmoil, despite the Andalites losing, despite the quantum virus, despite Aldrea's manipulations and Dak's self-doubt they still clung together. And not for a week, not for three years, but for long enough to fight the Yeerks, to procreate and to see the child grow at least a little bit. We don't know when they died, but I think we can safely say that they were together in some regard for longer than any other couple in the series (except Elfangor and Loren, but they had the nice suburban dream life). I dunno, I guess I just have an appreciation for love that survives through everything. Rachel and Tobias could have had that, I suppose, if Rachel hadn't died. Cassie and Jake didn't have that, though. The war eventually broke them.
