The issue I have with ranking Lupe high as others do as a pen is sometimes he gets so abstract and obscure with his references, and they're so loosely connected, that even if you get the reference, he can damn near say it means anything he wants and nobody would even know. This is why often times people dissect his bars and come away with multiple different meanings, and usually all of them are wrong. Lupe writes in a way (sometimes), that only he'd be able to fully explain what he actually means because it's so convoluted. He leaves a lot of ambiguity to his wordplay which to me isn't necessarily skillful as it is pretentious. He's a big Aesop Rock guy, who notoriously thrives on being vague with a lot of loosely connected wordplay.
And then outside of the times when he's trying to be grandiose, some of his wordplay is just corny. Some of his verses that are not overtly littered with inconsequent riddling, have bars that are just not good.
The highest level lyricists to me is able display high level technical skills (wordplay, scheming, metaphors, multi-syllabic rhyming), while being proficient and getting a POV across that is clear to the listener, even if they don't get every bar, they get the overall message the MC is relaying. If the only person that fully understands what you're writing is you, then you aren't as good as you think you are. You should be able to take a complex thought, write in your highly coded language (which rap is) while still making your messaging clear. That's what Nas does, that's what Rakim does, that's what Jay-Z does.
To me Lupe is able to do that in songs like Failure. When he's in that lane, it's beautiful. I have no complaints, it's elite.