If I had to answer, I would say either Misty or Serena, although thinking about Ash dating (never mind getting married) is strange because he never grows older.
With Misty I always felt like they had a sibling-like relationship, hence all the bickering. Misty was the youngest of four and her older sisters were awful to her, so it made sense that she picked on Ash and he did likewise- that's what her siblings do. If the show was entirely different and the characters were allowed to mature, maybe I could see them dating as teenagers.
Serena, she was actually interested in Ash romantically which seemed like more than a little crush, weird because they are young kids but if they had continued to become closer I imagine they would be like high school sweeethearts.
All the other female companions I feel would be friendzoned.