I'd say since it started -some would say that it started when Tupac was shot in New York, but for me, the label war started when Biggie signed to Bad Boy records :
94= East (ready to die and ILLMATIC!!!!!!!)
95= West (Me against the world + Hit'em Up)
96= West (all eyez on me + 2pac died : the Hottest year in hip-hop!!!!! : reasonable doubt; the score; the 7day theory)
97= East (Life after Death +Biggie died)
98= East (hard knock life + the miseducation of Lauryn Hill)
99= West (Eminem Joins Aftermath and creates Shady Aftermath)
2000= West ( Marshall Mathers LP)
2001= TIE ( the best year in hip hop after 1996 : chronic2001 on the west side and Blueprint and Stillmatic on the east side)
2002= West (paid tha cost to be tha boss- the worst year in hip hop)
2003 = East (eminem discovers 50 while Jay-Z drops the black album)
2004= East (Kanye west college dropout)
2005= East far far ahead (kanye west late registration and Game claims that he loves New York tho he's from LA + all the new beefs in New York and finally the "I declare war" concert)
2006= I was expecting that dre would drop "detox" and I would give it to the west but as it dosent seem like it's going to happen, we don't know yet,
Hold up!! Em and his flow, is a product of Detroit, and that's East Coast, or Midwest at the least. And while Tu Pac spent most of his career in the game in the West he was born, raised and started rapping on the East Coast (Brooklyn, NYC), and most knowledgeable folks know that his flow was clearly that of an East Coast Nicca. No other West Coast artist flowed like him, yet plenty of East Coast headz did and do.
He's a product of the East Coast, who moved to the West with his parents when he was younger, after attending a major school of performing arts in Baltimore. Yet, he later returned to the east and started his career with the Digital Underground, an legendary East Coast group. He rolled with them for a good spell shaping and and molding his talent, before releasing his own work, starting with "2Pacalyps Now".
2Pac was simply an East Coast artist repping the West. A trader, to say the least. Without an East Coaster (2Pac), the West wouldn't had even been in the same ball park as the East. East Coasters rap smart, eeeerbody else raps thug. Now one shouldn't ever get that Twisted.
Tu Pac, Kanye, Nas, Big, Rakim, Em, Common, Talib Kwale, Seagal, Big Daddy Kane, Public Enemy, Fresh Prince, Queen Latifah, Koo Mo Dee (SP), , Jam master flash, 50, Fabolous, Wu Tang Klan, Q-Tip, Fugees, Fat Joe, Busta Rymes, and on and on and on. The game is locked. The NYC alone, can stand alone by it self as the most profound spot in the world for Hip-Hip production in history.