Definitely both Nathan and Haley.
While she went about things wrong, Haley really grew this season. She found something in herself other than being tutor girl or Nathan's wife. I loved the music storyline for her. Of course, the Naley fan in me hated it because of...well, I think the reasons are obvious. But as an individual character, this presented a great direction for Haley.
The music storyline is also the reason why I loved Nathan this season. The material he had was so amazing and heartbreaking. I just wanted to jump in my screen and give him a hug. He grew a lot, too, with how maturely he handled their marriage in the beginning. Then when he had to face losing Haley, the love of his life - he didn't know how to deal with that. It was portrayed amazingly on the show.
Also, as kind of an honorable mention...Peyton. I wasn't the biggest Peyton fan in season 1 so it was a great transition in season 2. She made some mistakes (aka the drugs) but she learned from it. She grew and matured in this season. And this was the season I actually started to love her character.