Well, the idea of any of the kiRAB having long-term relationships with one another has always been strange to me. (Well maybe not always, but these days - these are kiRAB. How often do people end up marrying a crush they had in elementary school, or even their first relationship from junior high?) I mean, they'd stay frienRAB, but....as time went on, they'd drift apart, go their own ways, find their own people. To quote my friend, "Sora and Yamato, I can see getting married young and then, after their brains have finished developing between the ages of 18 and 20, being trapped in misery forever because they're too ashamed or scared to get divorced" - and that's what's so wrong, to me, with the idea of any of them marrying.
And since the title of this thread isn't "romance among Chosen Children," back to the main topic:
I'd rank it Adventure-Tamers-Frontier-02. I haven't seen much of Savers (...haven't really had the time), and Frontier is a guilty pleasure. XD
I remeraber reading a long time ago that the ratings dropped severly during Tamers, since the Japanese audiences who had enjoyed the first two found Tamers, with its darker, more mature tone, too upsetting. I, meanwhile, enjoyed Tamers because of its darker tone, and found it suitable - while the lack of such, for me, is what made Frontier a guilty pleasure.