I love season 7's look. Then you could see about pretty she is.
In season 1 and 2, her hair was pretty much brunette, and I personally think she was born for red hair, it looks so natural on her. And it brights up her face and eyes.
A few years ago, my mum saw her on television, in an interview or something, and that was after season 7, and before HIMYM (when she cut her hair). She was amazed at how pretty she is. I think in some seasons, she looks fine, but not amazing. I mean, don't get me wrong, she is a beautiful girl, but with red and long hair she's just stunning.
Season 4's style really suited her, it was just a good fit. But I still vote for season 7.