I believe your question is not, "Which is the hottest part of the candle flame," but rather, "which area around the candle flame produces the most easily captured heat energy?" These are two very different questions. The goal is to identify the ideal position for energy capture near the flame so that as much energy as possible can be transferred before being converted into either electricity or (more popularly) radiant heat. You obviously cannot insert an energy capture device inside the flame because the process of combustion itself would be interrupted. One popular solution is by the use of a clay pot combined with one or more burning candles placed so that the pot itself becomes a radiant heater.
The overall efficiency of this method can be determined (mostly) by the amount of space separating the pot and the candle since optimal energy capture will require optimal exposure to the flame. The question becomes, "What is the optimal distance between the energy capture device and the flame? I would suggest that the candle(s) be placed completely inside the clay pot as opposed to outside as is commonly seen in lamp shade configurations with this type of thermal heater. For obvious reasons, such an open air arrangement will achieve less energy capture than will a (nearly) closed system.
If you would like to see my simple design for a thermal candle-flame heater, you can send your request to me at firstname.lastname@example.org.