Which is the meaning of stable flow for you?

The stability of fluids is associated always with stratification and that is defined by the Richardson number.

Because I would like to compare two flows so I think stable is a good paramater for evaluating.

if I want to compare two flows (this fluid is better than another), what should I do?

I personally don't think, that the term "stable" is a scientifically defined term for characterizing flows. But if you apply this for the kind of flows which you like to study, so than what do you understand under the conditions, that these flows are becoming "unstable"?

I guess, that you are observing the one or other arising disturbance to your kind of flows. Disturbances can be quantified by developing frequencies and amplitudes of oscillations - not only in one place but in a larger number of monitoring/measurement locations. That could give you some quantitative measure for distinguishing one flow as more "stable" than another one, if oscillation amplitudes in those locations are smaller by amplitude and frequencies of such oscillations are smaller.

If that is not of what you are striving for, you need to define your understanding of "stable flow conditions" more precisely. An entirely transient laminar Von-Karman-Vortex Street of the flow behind a cylinder in cross-flow can be characterized as "stable" as well, in this case because the frequency and amplitude of the vortex shedding does almost not change over time for a given and constant laminar incident velocity. In that case, this flow is "wildly" oscillating but nevertheless "stable" in a certain sense.