The individual who has damaged hip hop the most is Eazy E. For creating the "gangsta" "reality" type rap that floods the market with wackness today, to completely commercialing rap, Eazy E has been the worst.

Speaking as a UK resident, the one man who has done most to advance the cause of Hip Hop is Tim Westwood, formerly of Capital Radio in London but now broadcasting across the country on Radio 1. The only national DJ to represent Hip Hop, he broke acts like Biggie Smalls, Cypress Hill, Brand Nubian etc while all the others were selling out to techno and jungle.
The one who's damaged the cause? Hammer and Vanilla Ice, for obvious reasons

The person who has done the most to advance hip hop is probably Dr. Dre. He is one of the few big names of rap that is willing to work with names from both coast.
The artist that has done the most to set hip hop back is Tupac. He is always talking like he is a revolutionary, but does nothing but smoke weed and talk a lot of shit about what he did with another man's wife.

Common Sense has done the most to further advance hip-hop
2Pac has done the most damage to hip-hop

KRS-One has done the most to further advance hip hop
2Pac with all his bad vibes has set hip hop back 10 years

Ice Cube's done the most to make rap better. He brought it out to the public without sellin out, every rap has a story behind it, mixed comedy and tradegy to practicality, versitile (hard or smooth gangsta or political),....

All the commercial ass rappers that seem to be more and more prevailant nowadays have hurt rap. Rap needs to stay away from Sprite commercials and exploitation movies