While I don't think 1998 was the last great year for music, it is probably the last classic year. At that time, a lot of (if not most of) the music had a purpose and a message. It was the last time radio had singles like this:
Aaliyah- Are You That Somebody
Monica- First Night
Lauryn Hill- Doo Wop
^^^ Those were huge singles. "Are You That Somebody" and "First Night" were rather innocent songs lyrically and thematically, but the production was hard. These days, these women (even the young women at the age of Aaliyah and Monica when those songs were made) are more ratchet and put it all on display. Even though there were songs like Destiny's Child's "With Me", it wasn't over the top or explicit.
Then Lauryn came in and slayed with "Doo Wop" without coming off preachy or self-righteous. These songs went too:
K-Ci & Jojo- All My Life (classic wedding song)
Xscape- Arms of the One
Mya- It's All About Me
Brian McKnight- Anytime
Jon B.- They Don't Know
The albums from that year were just
OutKast- Aquemini
Lauryn Hill- Miseducation of Lauryn Hill
Gangstarr- Moment of Truth
Jermaine Dupri- Life In 1472
R. Kelly- R. (the double disc album)
Playa- Cheers 2 U
Jay Z- Vol. 2
Juvenile- 400 Degreez
Big Pun- Capital Punishment
DMX- It's Dark and Hell Is Hot
No Limit had so much product on the streets that it's hard list, but Life or Death, Ghetto Fabulous, Charge It 2 Da Game, Give It 2 Em Raw, MP Da Last Don (mainly disc 1) were standouts from that crew though.