b) The voice recording feature would have the best s/n ratio on a flash based iPod, that is with the iPod nano line. Then again, iPod shuffle should remain the minimalistic mp3/aac player it already is.
Recording the FM radio would be the most comfortable with the hard disk based models. They have the capacity to meet the home recorder hobbyist's needs. However the internal hard drive's sounds would be a problem on voice recordings. Perhaps a line in recording instead would be better.
FM transmitter should be left out, since it only would make sure that those iPods would never appear here in Finland due to the transmitter regulatory issues.
A feature that wasn't listed at all, is the Ogg Vorbis playback and perhaps even recording. It's a shame that no iPod is capable of Ogg, the advanced patent free audio codec. For more information, cf. http://www.vorbis.com/
How Would You Improve the iPod?