River was set up to become an ultimate, awesome weapon or something along those lines, but never got there. She was a character that never came into her own, and was pretty dull. Lots of potential, but not very exciting without living up to it.

Inara was just. . .not very believable. I dunno, maybe I just don't care for the actress, but I never bought the supposed tension between her and Mal, was never particularly attracted to her in her Companion role, etc. Just didn't do it for me.

I thought about saying Simon, but it was fun to watch him come out of his shell, get it on with Kaylee, etc. I also thought about saying Zoe, but I thought she had a cool dynamic with Wash. She was obviously the dominant partner, but at the same time she was fiercely loyal and even, to an extent, submissive to him. Somewhat. They avoided the "strong willed woman browbeats man into doing whatever she wants" stereotypical TV relationship, and they were good together, in a weird sort of way.

So! River or Inara.