Late 90's without a doubt.
Late 90's gave us Rakim's "The 18th Letter", KRS was still bringin' heat, The Wu dropped Forever, Tha Alkaholiks, Heiro, and 'nem was still active, OC, all of BCC, and way too many other lyricists were still heavy in the streets.
I mean, compare 90's Fat Joe to early 00's Fat Joe. Dude straight up dumbed it down after 2000.