This part by Kendrick Lamar is one of the best (parts) of a rap song ever.
I didn’t care much for the album Damn other than Humble and this middle part of XXX. But this part of the song is great.
I don’t know how to describe flow (is it the ability to make your voice match the speed and pitch of the beat?), but this song has it.
Eminem has it pretty good too, but I’m not sure what a good song is as an example.
Surprisingly Tom Green does pretty good in this part of a song.
I’m no expert, but I think “flow” for a rapper involves several good qualities at the same time, and can’t be reduced to just one criterion.
Eminem’s verse at the end of at the end of Forever was a masterpiece. It’s best they left it to last, because the other three verses could never hold up - anything coming afterwards would be a massive letdown. The entire song definitely contrasted all four styles, and, personally, Em destroyed the rest, whether you like his regular style and lyrics or not. Personally, his raps could easily be written with musical notation with virtually no fudging. Just count 1 2 3 4 on the beat, he will hit on the beat, with emphasis, and fit the lyrics.
Recently I’ve trended towards Snow Tha Product. Her style is much more “rapid fire”, reminding me of a young Busta Rhymes, without all the yelling. Outside of that, I haven’t listened to much recent rap. There’s so much junk out there that I run out of energy before finding something I like.
The original and one of the best masters of flow, Rakim, of Eric B. and Rakim.
Here’s Follow the Leader: Eric B. & Rakim - Follow The Leader (Official Music Video) - YouTube
Man, you hear the template right there: laid-back delivery, precise enunciation while staying chill as hell, hard internal rhymes, half-rhymes using the flow to sell them, a solid groove to work over.
This is 1988. Everyone learned from Rakim - Eminem, Snoop, Nas. Listen to Microphone Fiend, and Don’t Sweat the Technique.
And Rakim certainly influenced Q-Tip from A Tribe Called Quest.
Here’s Excursions, the opening track off their brilliant album, The Low End Theory: A Tribe Called Quest - Excursions - YouTube
Over a jazz-hook rhythm bed, Q-Tip just rocks us, laying out his story of learning to love music, and just immersing it in a stew of samples and beats. And the whole time he’s tell his story with a natural flow that pops, yet sounds like a story, not a forced poem. Really amazing. Q-Tip is the only guy who’s flow gives Snoop a run for the money in terms of laid back easy cool.
Yeah, it’s a bit like the term “swing” in that sense. When you say “that cat swings” or “Bobby’s a great drummer, but he just doesn’t swing” it’s not simply a statement on their ability to elongate the first note and shorten the second note of two-eighth-note groupings.
(And I personally tend to prefer “flow” that pushes and pulls against the beat and adds rhythmic interest, like Biggie’s chill, laid-back flow, for example. So it’s not specifically one type of rhythmic approach, like “the ability to make your voice match the speed and pitch of the beat.”)
I’m an Old School/“Middle” School guy, and AFAIC nobody does it like Grand Puba; when does that guy breath? Grand Puba -Thats how we move it - YouTube
Then there’s Big Daddy Kane, Black Moon, Brother Ali, Pharcyde, Redman (I love the part at the end, where it’s like he doesn’t even know what he’s saying), and a somewhat obscure old personal favorite of mine, Nine. He says “I flow like diarrhea when I’m droppin’ shit / Mamma mia, ain’t no cure for the pure lyrical gonorrhea” 
Thanks. I know a bit about Rakim but not much.
I went on spotify and downloaded both albums by Eric B and Rakim, I’ll check them out sometime.
I think Vince Staples has great flow. He has long verses that almost make you think he’s not taking a breath.
Jump off the Roof
Norf Norf
The incomparable, inscrutable Kool Keith - Supergalactic Lover and his alter-ego Dr. Octagon - Earth People. The guy has flow like no other, IMO. Both of those are over 20 years old, but he can still bring it yo: Dr. Octagon - Hollywood Tailswinging is from an album that was released 27 days ago.
Flow.
ETA: Goddamn I love this.