I think she looks best in OoT as an adult. I've always wanted to look like Zelda ever since...forever but I have more bronde hair which I guess Zelda never has had...I thought her hair was more brown in Twilight Princess.
Blonde Zelda has always felt perfect to me. Most likely it's because she was blonde in Ocarina of Time, and that was the first game I owned, and it's my favorite. I also tend to like blondes in real life too if that makes a difference.
A blonde Zelda is probably the best. That is how she has been for several Zelda games, and blonde fits her better for most of the artistic approach most Zelda games use.
I like brunette Princess Zelda and blond Princess Zelda is okay. Maybe in the future Princess Zelda can have diffrent kinds of hair colors along with Link, and Ganondorf.