Definitely Ran. She's been waiting for Shinichi's ruturn for too long to have Shinichi leave her for some other girl he doesn't know as well ( Shinichi would be the biggest jerk in the series if he did that). Also it may be true that Ran worries about Shinichi a little too much, but Shinichi has also been shown to worry about Ran just as much ( he's just a little bit more discreet about it).