I think you gotta grab Dempster first. He had a great 2d half last year and was doing well until he blew up for 2 games. Still, it was just 2 games.
After him, I think I'd go with Walker. Everybody keeps waiting for him to blow up, but it hasn't happened. I have him on both my teams and I plan on riding him until he does flame out. I just think you can't ignore his recent success, no matter how bad he was in the past.
After that I'd grab Putz. I believe Guadardo had another blown save last night?
I guess you didn't list Walker though, so nevermind about him. In short, I'd go with Dempster, then Putz, and then, only out of utter desperation, Rietsma.