If I was just going by the player and the impact this season, i would rank the possibilities as:

Bautista - yeah it is a fluke year, but MLB history is littered with guys that had fluky great years. Maybe he falls back to earth next season, but I think this season he continues what he has done for 4 months.

Willingham - He doesn't have the raw power of some other candidates, but he is a good hitter.

Hart - Sort of a hacker, not the greatest hitter but I think he would be an upgrade and he wouldn't be limited to LF.

The 3 guys above are upgrades over what we have now. Each of them would have a greater impact over the next two months than Burrell, Ishikawa, Rowand, Schierholtz.

Before I get to the rest of the list, lets throw in the cost discussion: We get additional years from Willingham and Hart so they will definitely cost major prospects/MLBers. And personally, I might not feel that great about Hart replicating his production next year.

Bautista would be a buy-high guy, but not as high as Hart would be and maybe we can stomach the price if they throw in a relief arm.

the others:

Guillen - a headcase, not that great a hitter or fielder anymore, but, IMO, he would be a slight upgrade over Burrell.

Cantu - not having a great year, and since he doesn't play the outfield, would take abs from Ishikawa and possibly Buster or Pablo. Would be kind of a non-move.

personally, if I am giving up prospects i would go after the guys that can help and are not necessarily being shopped, like Choo in Cleveland. Or Markakis in baltimore (maybe Luke Scott, too, but I would stick him in the above category).