Design of the low pass filter when decimating
You can determine the "best" filter by analyzing what type of errors is created and how much of each error your specific application can live with.

If your original content is already perfectly band limited, than you don't need a low pass filter at all and can directly throw away the extra samples.

The decimation low pass filter typically determines how the band limiting of the content is done. Here are the factors to consider


Aliasing: since the low pass filter is never perfect, the decimation will
create some amount of aliasing. The amount depends on the steepness of the filter and the energy content of the signal at the frequencies that need removing.
Pass band: The low pass filter isn't perfectly flat in the pass band so there is some amount of ripple you need to live with. The pass band must be below the new Nyquist frequency. The higher the filter order, the closer you can get and the wider the pass band will be.
Phase response: Any IIR filter will change the phase response of your signal. The steeper the filter, the higher the maximum group delay. If need to maintain the phase, you need to use a linear phase FIR filter (ideally in a polyphase configuration).


The choice of filter is a trade off between pass band width, pass band amplitude distortion, pass band phase distortion, aliasing noise and computational complexity.If you have a spec for all of these, you can design the "best" filter. An example would be "pass band +-0.5dB up to 90% Nyquist, aliasing at -80dB or below, linear phase".