I thought I already replied to this, but, from experience, I like these guys located at: Public Aerodrome Gomez - Route 2 Km 65 - La PLata - Buenos Aires - Argentina Tel: (011) 15 5249-9282 / Exterior: (+ 54 11 9) 5249-9282 Nextel: 54*397*4058.
The place is called the fly ranch.
There are two outfits, one that specializes in hang gliding (my favorite) and one skydiving. When you call mention you specifically want the info on skydiving, which they do as tandem also.
I go there whenever the winds allow, and they are a great group of pple, and are serious about safety.