Hmm tough one.
I never really vibed with Fergie although her music is catchy. It just rarely makes me go back to her songs. Glamorous and Big Girls Don't Cry bring the nostalgia for me.
Gwen was big at the time, I remember. Sweet Escape, Cool, 4 in the morning, Hollaback Girl, Rich Girl. However, these songs are from the span of 3-4 years (2004-2007) so I wouldn't say Gwen captures the 2006 sound.
For me, it has to be Nelly. Nelly & Timbaland & JT were such a major trio. I love Timbaland's production from the time and for me that's the sound of 2006/2007. Promiscuous, Say It Right, Maneater, All Good Things, Give it to me, Morning after Dark. I go back to her songs the most out of the three. It's super nostalgic and each song still slaps. I'm hoping they make a comeback.