The popular response is going to be Selina, and for very good reasons. For me though it's always been Talia because of the "forbidden love" type aspect of their relationship dynamic. The both are extremely attracted to each other, Talia loves Bruce, Bruce probably loves her to some degree, but can't give in to his feelings because he doesn't support her father's views or actions and she has always been too loyal to Ra's.
Also a secondary choice for me would be Zee. I've always liked the idea of Bruce and Zatanna together.
@Guy...Talia and Jason only did it that one time. Don't think they've had any contact with each other since...and knowing DC editorial, that's probably not even canon now lol.