FFT - preferred way of tone detection
Number 4 : Comparing narrow-band energy to the energy of the total spectrum is a commonly described method of DTMF detection.  This may involve using more than one FFT result bin and/or interpolation between bins. 

The other methods depend on whether one has an accurate estimate of the signal level, noise floor, and how well the tone frequency is centered in a single FFT bin (related to frequency, sample rate, FFT length and window).