The systems I always liked were, Dragon Age 2 is by far the best romance system from any game with romances I've ever played. I loved being in a rival romance, because even with this name, most of the time it led to an even more healthy relationship.
DOS2 also had an amazing system, where sometimes a single gesture, like a hug, holding hands, or asking if they were fine would lead to a growing relationship. It felt natural, most of them anyway.