People say it's Latias b/c Bianca wears a hat, but in Bianca's room, you could see the hat on the easel, which means Bianca wasn't wearing it then. Also, Latias doesn't paint, right? Why would she give him something Bianca made as a good-bye gift? Furthermore, when the camera zooms in on Bianca's room, you can see a hand grabbing a painting of an easel, indicating that Bianca HADN'T left to go the the market yet. Unless Latias was up in Bianca's room, but why the heck would she be there? So logically speaking, I would say Bianca.
However, Latias seemed to like Ash more. Also, she didn't say anything to Ash, she just ran up to him, kissed him, gave him the drawing, and ran off. Obviously, that would make it seem more like Latias. Perhaps Bianca was trying to trick him...? And I also heard that in the original, Japanese version, Satoshi thinks to himself that he knows it was Ratiasu, and if so he was probably right. However, I haven't seen it in Japanese, so I have no idea if that's true or just a rumor.
Basically, unless someone can prove that rumor true, I'm going with Bianca. But that's just my opinion.