The key to a 12th servo is that it is reliable and it centres well. That usually rules out anything other than the Futaba or Sanwa servos. The Core seems good (I used one in a GT12) but its spindle position means it won't fit the majority of 12th chassis'.
Futaba is by far the best choice. S9650 or its latest replacement the 9760 (I think). Hitecs suffer from centring problems in the medium term and Savox are too power-hungry for a 12th installation using 1S - it will rob you of power at best, and cause shutdowns at worst.
People will disagree with this, but as any review of servos in a 12th meeting will tell you, Futaba and Sanwa are there by the scores; the others not so much to not at all! HTH
