Downscaling raster maps

Q: What is be the recommended way to downsample a raster map?

A: The "best" way is subjective, but the main options are r.resamp.interp, r.resamp.stats, r.resamp.rst, r.mfilter.fp, or any combination of those.

Another option is to use i.fft followed by i.ifft with a mask applied. However, that requires that your raster contains no null cells (so you may need to use e.g. r.fillnulls first), and it also requires that you can fit the entire raster in memory (so it isn't suitable for very high resolution data).