Since these are mostly reliable types, with no one guy standing head and shoulders above the rest, Polanco is probably the best across the board guy, even though that Philly playing time situation is questionable.
I would gamble and try to catch lightning in a bottle with Cantu. He's off to a great start, hit pretty well last year in an impressive trial, and he's also eligible at 3B. Could end up being a guy that's worth holding onto.
Spivey also has potential to do well. Hits lefties very well, and if he can stay healthy (??) not a bad guy hitting second for Milwaukee.
I'm not high on Durham as he is always injured and like all the Giants, his value is decreased with no Bonds in the lineup. Iguchi has solid credentials, but I'm wary of Japanese guys in their first year stateside. Tough adjustment.
Walker is fine, will get you BA and a little pop. Alfonzo is okay for BA but little else, Infante is too shaky in the BA category, and Young is of little use.
So I'll say go with the short term gamble on Cantu.