I have to disagree about the "longest history" thing, from Misty's side. She seems closer to Tracey now. She was with Ash from Kanto to Johto, but since then, he's seen her maybe... 4 - 5 times? [And he's never once gone to visit her specifically.]
But while he's been away [Hoenn to Sinnoh], she's just a short walk bike ride away from Tracey, and he's been shown to visit her on multiple occasions. I think it's safe to assume he's been visiting her somewhat regularly since then, as well -- I'm sure her Azurill likes to spend time with its mother.
So, the way I see it, he has indeed been with her the longest(Technically Brock, but...yeah), but Tracey is quickly filling that position for her. They've both spent the time it takes to get through 2 full and one small region. So, unless Ash starts spending some more time with her...
I haven't seen much of this "romantic" history, either. There was that one dance or whatever, and... I'm not sure what else. Care to refresh my memory? I seem to only remember the bad things. Such as Totodile. :\ I mean really, he couldn't let her have that one pokemon? That was the moment that any hope of Pokeshipping died for me. Also, I love how he invited her back to travel with him after her sisters got back from their trip... oh, wait...