WTA: Wozniacki, because she's a happy person and she values many things I also value. Plus, I would be able to speak Swedish with her.
ATP: Err... if I had to choose, maybe Djokovic, because he seems to really enjoy life.
We should be appreciative of Federer though, because we will never see anything like him again. The fact he can still compete for slams ever after the erosion of his physical skills is really a greater testament to the natural talent he has than the period of time when he was dominating the tour.