"N.Y. State of Mind". And not just the lyrics and delivery; according to an article concerning the making of Illmatic, Nas truly did not know how he should launch into the song proper (hence him saying "I don't know how to start this shit" at the beginning of the song), but when notified that the mic was live in the middle of recording, charged straight into the first verse. And that was the cut they kept in the final product.
"Ether". The song gets especially awesome in its third verse, largely considered the best diss verse of all time.