This is so hard to decide... on one hand I want to pick Girlfriend because it was the one of the first songs I played attention to... but I want to vote for When You're Gone too cause it's a very good song.
Is there a both option? No? I'll choose When You're Gone then.
BTW, Avril didn't steal the Girlfriend lyrics from someone else, she was drunk when she wrote it. She actually said that on the radio. Rumors get good singers in so much trouble now.