This was a pivotal era for me and a lot of my peers. It was like when my peers first started downloading heavily, long before we ever thought about the consequences it was having for the music industry. In a sense it was a time when it was very easy to get into any style of music. Since we were teenagers, a lot of superficial stuff (rebellion, wanting to be different and/or wanting to fit in) weighed to some extent on a lot of the stuff people chose to get into.

On The Radio/Mainstream front:
I'm inclined to think Usher and Lil Jon's "Yeah" was perhaps the biggest hit of this period in my area (Florida). I still can't take that song in any form, including being drunk as hell at the club lol. I hate that beat so deeply.

I remember when 50 Cent's "In The Club" dropped and he officially became every high school age hip-hop fan's favorite rapper. I played the shit out of Get Rich or Die Trying and really thought he'd be a long standing, respect figure of hip-hop. I guess half of that is true lol.

I also remember when Kanye's "Gold Digger" dropped. That one I loved from the beginning and didn't mind being played at every Homecoming and Prom.

Being in the south, of course Young Jeezy and T.I. really took off and where bumped at house parties regularly. I didn't like Jeezy then 'cause "Soul Survivor" was the big one getting play and that's up there with Usher's "Yeah" for me. As for T.I., shit I still get a little hyped for "What You Know".

Underground/Personal Picks:
Hell this is the era where I discovered "Undergound hip-hop". I remember Cage's Hell's Winter really helping me through all that teenage angst. Same with a lot of Murs and Atmosphere.

Cannibal Ox opened my mind to out there hip-hop and El-P's production. Shit like this blew my mind back then http://www.youtube.com/watch?v=pADgllloQg4 and showed me just how different underground and mainstream hip-hop were in that era.

Aesop Rock did the same with Labor Days (I was a couple of years late) in regards to out there lyrical styles.
A friend put me on to Ghostface Killah's Pretty Toney album. It's still my favorite (I know I'm in the minority) and I just never felt like he had the kind of energy he had on songs like this http://www.youtube.com/watch?v=hO7NeKdidEc&feature=related on any of that Fishscale and later shit.