FIR filter design with flat passband but equi-ripple stop-band
I suggest you the Parks-McClellan method. In Matlab you can use $\tt{firpm}$.

Matlab's (now obsolete function) $\tt{remez}$ also uses this scheme.

The FIR filter is optimally designed to approximate e.g. a Chebyshev filter such that the maximum error between the filter's response and the desired response is minimized.

For more information see this and the examples therein.