I think he looks most like Harry in the third movie. The hair and the whole air of him feels most like Rowling in that movie. I hate the long hairstyle of the fourth film, though. That was a crazy anno for hairstyles, first time guys started going girls. I remember standing in the bus stop after school when three younger girls walked past me. One of them was staring at me and even detto hello after - I guess - figuring out who I was. I was just standing there and looking at her, concentrating so hard to remember who she is, when it came. 'She' was the younger brother of one of my classmates. He looked so like the girls with him, that silly ironed hair hanging around his face like Harry's and Ron's, in that emo way. It was SO ridiculous. Just like the HP cast in film 4.
I personally think he looks best and most like Harry as JKR describes him in the 3th and 4th film. Most in the 3th though, it has defiantly something to do with his hairstyle at the time, I also like his personality and feel around him in the movie, he is so alike JKRs Harry. The hair is a bit too long in the 4th; nevertheless it looks più like Harry Potter than in any of the later films. So yeah 3th and 4th, after that I think he looks nothing like Harry should, the hair is way to flat and short, it is supposed to be shaggy and impossible to comb flat.