I'm not going to go with worst. Because it's different for me. I was born in the 90's, so I wasn't around to hear the terrible Hip-Hop of the 90's because I didn't start listening until the late 90's/early 2000's. And bad music doesn't stand the test of time. So I haven't heard a whole lot of truly terrible music from the period.
There are, however, plenty of rappers from the period I dislike. There's a reason as to why you don't hear about the terrible rappers from the period. It was so difficult for them to become famous with the lack of Internet (which is a also sometimes a perk for today's music). If someone was terrible, they just didn't make it. Period. And if they did, they were forgotten over the last twenty years.
The main one from the period that I cannot stand is Biggie Smalls. Sorry.