As usual, the free agent options are limited in center field.
The best bat among the available CFs is Javier Alvarez. While he managed a line of .282 / .369 / .459, excellent for a CF, Alvarez doesn't possess the range most teams require ( 12 '-' plays). Alvarez can still expect a strong market however; he profiles as a serviceable SS or an elite defender at 3B.
Harry Rios (player option) has the range to play CF, but has difficulty tracking flyballs. For many teams, Rios' (.276/ .336/ .429) line and 3 '+' plays will not outweigh his 16 errors.
The best defensive option is Morris Davis (.253/ .318/ .425), although he hasn't actually played in CF since season 11. While Davis' main attributes are his huge arm and great range, his bat also profiles nicely at the position.
Teams with an appetite for a project could take a look at Willis Roth. Banished to HiA by the Cleveland organization for Season 13, Roth is only 33 in Season 14 and he would still be valuable if he is capable of performing near his career averages ( .272/ .334/ .322 ). Roth would be a defensive upgrade for a few clubs, but is probably better suited for 2B.