At its best transmission quality, what is the equivalent samplerate and bitdepth of FM radio?
There isn't a single, simple answer for this question. This forum post explains:
If you're looking for a compression bitrate to use, the answer gets even more complicated, as it depends on the compression format and algorithm you're using. However, I found a table of recommended bitrates for various source media. From this, you can see that for analog sources such as FM radio, analog tape or LPs, and an MP3 target, the author recommends 92-128kbps.
And see the audacity wiki for an explanation of FM sample rates. In summary, you can use a 32 kHz sampling rate with "only a little quality loss."