All of them read the same data, with the only thing changing being Tj Max. There is no difference in the calculations besides one number.
Core temperature = (Tjunction Max - DTS reading)
By simply doing 1st grade maths and changing Tj Max you can achieve the same readings from all these programs. Therefore none of them are really any more or less accurate.
Real Temp does let you calibrate the idle temps while no other software can do that, but most people don't bother which negates any "improved" accuracy that Real Temp could have. Speedfan allows calibration of TCase as well which is why it and Real Temp are preferable to Core Temp and Everest.
Read Comp's sticky as well, it's there to be read