So I understand I am necromacing this thread from quite awhile ago but I'm bored.
I realized this was a pretty long post and felt the need to toss a disclaimer up here that some of the songs discussed below are explicit and almost all have adult themes, as such they should be listened to with appropriate discretion.
I think the questions posed in the op are actually pretty interesting. First off I don't really think there is such a thing as "good" or "bad" music. Obviously there is a collective taste that the audience of a genre of music enjoys and that is usually what determines a "good" song from a "bad" one. As far as rap goes in the past lyricism was king, poetry put to a tune. But in current days beat fetishism has taken over mainstream rap and it's mostly about production value and beats that "move" you.
My favorite rapper and one who I think does a good job of blendig clever poetic lyrics with really diverse beats is childish gambino. It's infortunate his sophomore album "because the internet" is his most popular because camp and awaken my love are both clearly albums he spent more time with and poured more of himself into. Redbone/california/zombies are really really fun songs that can really work their way inside your head and make you never want to stop listening, they also have a very gorillaz vibe to them if you liked them.
B.o.b. Is also a really good example of a poetic diverse rapper who has everything from club bangers like "bands" to introspective ballads like bob vs. bobby ray.
As for "bad" rap I guess Big baby dram comes to mind. Broccoli and cash machine are songs that are really fun to listen to and have catchy beats that make you wanna move but the beats don't have any real progression to them nor do the lyrics have any meaning to them whatsoever.
Lastly if your a fan of early 2000's rap at all T-pains NPR tiny desk concert on YouTube is a MUST WATCH I'm not going to spoil anything but really go listen to it.