I'm very torn on this...but I voted for Dre. The man can do it all... produce, rap, find talent, you name it. The Chronic is one of the legendary rap albums, the man brought us Snoop, Eminem, Ice Cube, etc.
If it's straight rapping, not counting anything else, it's hard to argue against 2pac. the guy had some incredible verses, could rap about anything and everything. But for overall contribution and impact, Dr.Dre gets my vote.
I am a bit surprised for Ludacris to be brought up... I don't think he's much of anything in the rap game. I don't think much of 50 cent either, but that's just my opinion.
Another name which should at least be brought up is KRS-One...he won't get any votes but he should at least be an option.
JD is very underrated as both a rapper and a producer/talent guy.