Frequency modulation is best achieved at what stage in a transmitter?
At an oscillator (first RF stage).
FM transmitters generally modulate an RF oscillator by pulling its frequency slightly above and below its center (unmodulated) frequency, in correspondence with the modulating signal.
This is often done using a varicap (variable capacitance) diode connected to a crystal oscillator, or a VCO (voltage controlled oscillator).
The frequency modulated output from the oscillator is then either amplified for transmission, or 'mixed' to the required transmitter output frequency and then amplified for transmission.