transfer function estimate better approximation

I am using transfer function to eatimate the natural freuncy of this system, where there was an output signal for a certain input signal.

Can i take the peak near 1.6Hz in this plot as the natural freucny of the system or do I have to do curve fitting ro estimate the same.

Further, is there any guidelines for curve fitting? I am using sum of sines, as it gives me a bvery smooth curve fitting.

Thanks in advance.