Oh, definitely Lucas and Haley - no question about it. Everything about their friendship was amazing. They were supportive yet they were always honest with one another. And I LOVED that it was always platonic between them. Hey, I'm a girl who loves the classic "best friends falling in love" as much as the next girl, but it's so cliched. If the writers had chosen to "test" that route between Laley then I probably wouldn't have loved them as much as I did/do. I was so ecstatic to see Lucas once again in season 9, and how great was it that all of his scenes were with Haley?! That was pure perfection.
I honestly didn't mind Haley and Brooke, but seasons 8-9 kind of ruined that love feeling I had for them. The friendship was shown onscreen to be very one-sided and kind of forced at the audience in the aftermath of Leyton's departure.