personally I like Mayberry, Belt and Parnell (in that order).
Mayberry is very risky, as there's only 1 open OF spot for him and Domonic Brown to fight over. However, he's hit really well this year. If you have to decide soon I would try to trade him, but if you can wait until Spring Training, he's someone who would be a great value IF he started.
Belt will likely start in the San Francisco OF next year, and if he's guaranteed a starting spot could go somewhere in the 20's in an auction (IMO). I understand he wasn't great this year, but he hit well (again) in AAA, and was jerked around in the majors.
With Parnell its all about if he gets the closer role, so like Mayberry you'd have to REALLY believe that his skills will earn him the job to keep him now, but if he is given the job will likely be worth significantly more than 9.
Delgado's lack of K's concerns me, so from a skill standpoint I prefer Vizcaino. However, Vizcaino could be put in the bullpen again next year.