By a long-shot... Bale. Maybe it's because I play with Tottenham a lot but Bale on the left, Lennon on the right, Dembele and Sandro through the middle. It's just a perfect balance for a player like Bale who has height, strength, pace and power. Also can be played as a CF.
The best players have to be players such as: Welliton, Emenike, Eto'o etc. Even though they aren't amazing in real life, and their stats aren't the best, it's because they have pace. Fifa 13 is all about pace. This makes players with high pace the best players. Especially Emenike because of his pace and strength