How do we define resonant frequency for a circuit, that has arbitrary combination of resistors,capacitors and inductors?
Here comes the commonly accepted definition for resonance:

The input-output relation (V-I, V-V, I-V or I-I) of a frequency-dependent circuit shows amplitude and phase relations which changes when the frequency is varying.

The frequency, where the phase difference between the two quantities is zero is defined as a "resonant point".

In some cases, this "resonant frequency" gives a maximum or a minimum of the corresponding output-to-input relation - but not always. Therefore, these amplitude considerations cannot be used as a general definition for resonance. It is the phase relation between input and output which is used for this definition.,