Actually, I've found that the best, in order, is:
• Direct Line
• Cassette
• FM Adaptor
The adaptors are the most unreliable, even if you are able to find an "empty" station.
If you are lucky enough to be located outside of a major metropolitan area,
then your reception will, at best, be only good and not great.
After you get what you think is the best reception, try listening to solo piano music -
it often has the sound of ring modulated tones.
Possibly interesting if you're listening to Stockhausen's "Mantra,"
but a real bugger if you're groovin' to Tatum.
Again, if you're, for example, stuck with a rental car without a cassette deck,
then, I'd not recommend the Belkin FM gadget, but rather, "Air Talk."
Also, "Air Talk" has the added advantage of not covering the complete top of your iPod,
so you can have access to the "Hold" button if you need to use this.
The Kenwood that Soulstation mentioned may be a groovy possibility!
just a suggestion...