The lowest frequency!

You should use the same speed as the sampling rate(Fs) of your input signal.

Your cutoff point is then related to that Fs. For example if a lowpass filter cutoff is 0.25Fs and your signal rate is 12MHz then the cutoff will be 3MHz meaning all frequencies from 0 to 3MHz are passed.

your frequency may be labelled in terms of Fs ratio or in radians(0 ~ pi) where pi means .5Fs

For more advanced designers, you can pass your input signal to a higher or lower data rate(interpolation/decimation)