Dre is a genius.
Everybody else is several notches below him. Not sure I would be a fan of rap music were it not for Dre's influence.
From NWA to Snoop to Em to 50 to Xzibit to 2Pac... Dre is a fucking master.
You could combine Jay Z, Piff Dinky Duddy (or whatever he's calling himself these days), Master P, Biggie, and DMX and not get a 10th of Dre's brilliance.
The guy is the king.
Dre's awesome, no doubt...but not at rapping. Producing? Awesome. Rapping? One over par.
Sure, there's some stuff that Dre sounded pretty good on when it came to rapping. "Forgot About Dre" comes to mind. But, of course, that style and type of flow is not typical of Dre; this is explained by the fact that Eminem wrote most of the lyrics with help from a couple of other guys. In fact, Dre doesn't write his own lyrics. Jay-Z writes some of his stuff, ironically, such as his verses in "Still D.R.E."
So, no, I wouldn't compare Dre's rapping abilities to Biggie or Jay-Z. I would, however, say that Dre is one of the best producers in hip hop. Also up there with him is The RZA, DJ Premiere, The Neptunes, Swizz Beats, and Timbaland.