Describing the stop bandwidth of a filter using S11 and S12 parameters

It basically depends on the application. S21 provides the description of the filter, while S11 (matching) in the stop is upon the application specifications.

So if i want to describe the bandwidth of a filter, should it be describe in term of S11 or S2

Both.
In an ideal two-port loss-free network |S11|^2=1-|S21|^2.
You must just check the band in which the |S11| is below a specified value
(e.g. |S11| < -26 dB).
That range of frequency is the passing band.

Filter bandwidth is defined by S21. To match filter in a circuit you should use a filter by low S11, (S11 ~ 0). I agree with Aleksej.

s11 characterizes the filter when looking at the input. s21 is a characterization of the through and insertion loss. To add more confusion, there are numerous definitions of bandwidth that are often applied. Decide which one you want.

I think the bandwidth be measured in S21 similar as group delay