Best
Snoop Doggy Dogg went from being feared gangsta rapper to Snoop Dogg hanging with Martha Stewart and hosting the Paris Olympics. Musically, while he struggled to find himself after Doggystyle, he was hit and miss, but hit his stride when linking with The Neptunes.
Kanye West went from basement producer to super producer to iconic Hip Hop superstar. Watching him go from the Talib Kweli/Mos Def/Def Poetry lane to the Jay Z/Wayne level was something else.
OutKast reinventing themselves with each album. 1994-2004 OutKast was really something to see.
Nas is the same deal. Nas was reinventing himself a lot as well. There was a huge transition from Illmatic to It Was Written. Then to go from the I Am/Nastradamus phase to the Stillmatic/Lost Tapes/God's Son era to the more experimental Street's Disciple/Hip Hop Is Dead/Untitled/Distant Relatives era to Life Is Good to Nasir/Lost Tapes 2 to the 6 albums with Hitboy is CRAZY.
Missy Elliott is another one who reinvented herself with each album.
WORST
Puff- he was onto something with No Way Out. The team of Mase, The Lox, Biggie, Black Rob, and Lil' Kim hid his weaknesses as an artist. When he dropped Forever, all of his weaknesses were exposed because only Lil' Kim was featured and while that album had a crazy guest lineup, Puff was in the forefront. It was never the same after that no matter how many times he changed his name.