Here's my two cents.
Personally, I think her favorite flavor was vanilla. I don't know why, but it seems so... American, I guess. Like Ginger, who was once referred to as being "as American as apple pie." So it just seems to fit her.
Another reason I think this could be because of these photos:
I know they're in black in white, but it still looks like vanilla to me. The second photo for certain. Now that I look back at the first picture though, it does look a little more like chocolate, but that could just the lighting or shadow or something. Either way, I'm still sticking with vanilla.
But then again, since Ginger loved ice cream so much, maybe she didn't have a favorite, maybe she loved them all. As long as she had ice cream, she was happy!