OVERRATED:
1 Eminem, this dude is fucking shit. worst voice to liten to, sounds likea bitch and talks about shit that is not funny anymore.
2 50 cent, wow. Not 1 song he has made is good and he sounds like a slack jawed yokel.
3 Kanye west, pure garabge. dont know how people listen to his shit, but then again, hes mtv
4 lil wayne, another garabge wack rapper talking about nothing
5 snoop, after doggystole, everything from there was/is garbage.
6 all the new fad/dance craze/gimmick rappers, these dudes are a dime a dozen.
UNDERRATED:
Tech N9ne, probably the best flow of any rapper, very versatile and plain old good to listen to.
C Bo, because most of the world have not heard him, he is a beast on the mic. (new shit with young buck is shit though, sorry)
Twista, because of his old shit, its fire, and all the chicago rappers he made music with got talent, he went commercial in recent times, but he is a very good rapper.
Mos Def, he is rated highly, but not high enough imho, he is what most people need to listen to before they hear anything else, because he is the epitome of hip-hop.
Theres alot of others who can be on the list of underrated, but I think these stated are the most obvious.