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:
You can estimate the approximate Audio information  content of FM by using the bandwidth and dynamic range. FM pretty much tops out at about 15K and the dynamic range is about 70db. Each bit represents approx 6db so we need just under 12 bits and a sampling rate of 30K.
Thus = 12 x 30,000 x 2 (2 channels) = 720Kbps (CD is 1411kbps) that is uncompressed.
- FM transmission is somewhat more complex than that with carriers and what have you but we are not interested in that level of accuracy.
This question is also complicated by the fact that virtually all radio stations nowadays use lossy compression extensively in their production process. So from a quality standpoint, it really could be far worse than you might think.
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."