Biggie - He was a Stretch Armstrong show fixture, had a bunch of guest appearances and mixtape joints before he blew up. He started getting popular with the Mary J Blige and Craig Mack remixes, but "Juicy" is what blew him up.
Nas - Also a Stretch Armstrong fixture. He was down with Main Source and Wu-Tang. It Was Written blew him up. He was huge in the underground (see Illmatic)
Xzibit - Never a dominant figure in the underground. He was down with the Likwits. "What You See Is What You Get" and "Paparazzi" were underground hits. "Bitch Please" blew him up and put him into the mainstream.
Jay-Z - Rolled with Jaz, Big Daddy Kane, Mic Geronimo, Big L... Jay-Z was in the underground for a while. "Dead Presidents" got him a lot of shine for the Nas sample. "Ain't No Nigga" put him into the mainstream mix, but "Hard Knock Life" is what made him a star.
Snoop - Never was an underground star. No one had heard of him before "Deep Cover", and after that, well, was The Chronic.
Dr. Dre - He was obscure, and then there was NWA.
Eminem - He dominated on the battle circuit, rolled with the Outsidaz and Rawkus for a while. "My Name Is" is obviously what broke him out.
Wu-Tang - RZA and GZA both dropped solo shit that went nowhere. They got a lot of underground buzz with "Protect Ya Neck". RZA did the Gravediggaz shit. "C.R.E.A.M." blew them up.
Mobb Deep - they stayed underground. "Shook Ones, Pt. II" and "Quiet Storm" were popular songs, but who but hip-hop heads really remembers them? They never really had mainstream hits, though they've gone platinum, I'm pretty sure. I guess that's why they roll with 50 now.
Ja Rule - He was down with Mic Geronimo and Jay-Z. He caught mainstream hype with Jay-Z's "Can I Get A...", but he didn't blow up until that joint with Lil Mo.
50 - Probably one of the most popular underground rappers ever. "Wanksta" went from an underground "Hey Ya" to a mainstream hit, and then "In Da Club" blew him up.
DMX - I know he was on the scene, but don't know too much about what he was doing. He blew up with the Lox's help, then put them under Ruff Ryders. "Get At Me Dog" blew him up, but "Party Up" was his big hit.