Lopez is a full-time ss whose value could take a jump if his move to the second lineup spot is permanent. He's young, with a high potential which he unfortuately won't realize this year. Two things to watch: walks and steals. If he develops a little more patience at the plate, and starts taking more risks on the basepaths, he'll become very good. A nice keeper, but you can do better this year.
Eckstein won't give you any power (the 9 rbi in two games were a major fluke), but his average should improve to around .300 (he has great bat speed and hardly whiffs), and his steals are valuable.
Guzman could well be the best of the lot, but his knee trouble worries me. I'd rather let someone else take that risk.
I agree that Eckstein will give you the best value, unless you're looking primarily for power. Eckstein's surge won't last, but he'll still be a good fantasy option.