Last time I checked, "Bold & Delicious" was (and possibly still is) my most played song by Ayumi. (It also holds the #9 spot in my Top 25 Most Played on iTunes.) I think the reason for this is, other than the fact that I love this song, that I constantly played it when I traveled to New York with my school choir in 2006. "Bold & Delicious" had just been released and I guess I kept thinking of how the music video was shot in New York, so I thought it just suited the whole occasion!
Other songs I constantly play and purposely pick would be Naturally and BLUE BIRD, both of which linger around in the Top 25 playlist on iTunes.