I'd say probably Remlinger. Farnsworth, when he's on, is a picture-perfect setup man, and I think the Cubs would like to use him as one. Hopefully he'll get over whatever was messing him up last year.
As a long shot, I would be only mildly surprised to see Juan Cruz get an opportunity at closer. He's got the stuff, and one of the primary concerns about him is his stamina in long games, which disappears when he's only asked to pitch one inning.