Not a “song” per se, but Black Thought freestyling effortlessly for like 10 minutes straight is impressive AF: https://youtu.be/prmQgSpV3fA

For context, that show asks everybody to freestyle and some refuse, some do a few bars, but nobody goes for 10 minutes, lol.

For an actual song, and you wanna talk art, Blackalicious - Attica Black. Really love the sampling and the musicality of it, there are so many layers and it’s fun to listen to. https://youtu.be/qHRxorG8ABs