My gut says it has to be Aramis Ramirez. Ensberg is another guy with a high ceiling, but in Wrigley, with that lineup, I see big things from Aramis this year. He may end up being a top 5 3B. As a matter of fact, I wouldn't be surprised if the Cubs lineup produced like last years Braves.
With Patterson, Sosa, Lee, Ramirez, Alou, Walker protecting each other, the results could be scary.
Of course, they have to stay healthy.