Hmm... hard to say really. I kind of play my courier as someone, who is afraid of getting attached to anyone (Legion Hitsquads and what not).
However, if I had to choose:
1.Veronica is great, she is witty and I enjoy her company a lot. She is a lot like a younger sister to my courier and that is why I cannot bring myself to do anything else with her.
2.Cass is also great, who doesn't love redheads? She had plenty of dialogue, before she got way too silent in one of the patches. A good drinking girl, but not a love of life.
Damn, I like all the characters in New Vegas, but I haven't found anyone that would suit my courier's tastes.