Damn man there's some dumb posts in this thread.
To sum it up - Rap music has changed heavily in the last 10 years, it has gone from being a small part of music sales to be one of the biggest. If you group RnB and Rap together it probably accounts for the biggest sales worldwide (because Country is so huge in US), it is definately the quickest growing. Back in 1994 very few people listened to hip-hop, and it got played on select "specialist" stations. Now it is played on 80% of stations even the most commercial stations, so therefore the artists are reaching many more listeners than ever before.
Considor 2pac alive now, living his life like he did 94-95, he'd be front page news world-wide for all the shit he pulled, all the shit he didn't but he got blamed for, and then for the other little things he did - films and music. But he didn't have the influence on the music as large as Snoop and Dre did back in the DR days, although obviously this is mainly my opinion. As for Eminem he is easily the "biggest artist", due to the fact he is around and making his best (excluding encore) music at the time of rap musics peak.
If all 3 were around at the same time I'd probably say 2pac would be the biggest star, he managed to cross over and make mainstream pop songs and still be fairly well respected, and with all the media attention he got just marginally out does Eminem, for newspaper inches relative to popularity of hip-hop. As for Snoop he helped build DR but it was mainly down to Dre's vision.