It's gotta be a classic for a few reasons.
1. Everyone has heard it. Ain't no way in hell you listen to hip hop and HAVEN'T heard Stillmatic.
2. The album came out at a very pivotal point in Nas's career. If not for this album Nas might have been another in a long line of rap casualties. Had a "spark when he started" but was on the fast track to irrelevancy. Instead he drops Stillmatic and new life is breathed back into his career and it's allowed him to go strong even to this day.
3. It's home to Ether... arguably the most infamous diss track of all time. Also one of Nas's defining songs.
4. It's home to One Mic, another one of Nas's defining songs.
5. It's just a damn good album. It had such a fresh feel comin off Nastradamus. It was gritty but it was clean too. And Nas as an MC really came off with a new found energy we hadn't really heard from him before. Listen to that intro man... you FEEL Nas's electricity on that track. The passion, the energy, the soul...

Plus there's just so many great tracks on there. Got Urself a Gun, Rewind, Second Childhood, What Goes Around, Destroy and Rebuild... it really is a monster album. So many great songs.
Definitely a classic.