The most iconic passage in Chukat for Rashi is the command at Bamidbar 20:8, where Moshe is told to speak to the rock, paired immediately with the divine verdict at Bamidbar 20:12 — 'because you did not trust Me enough to affirm My sanctity' — which together form Rashi's central discussion of why Moshe and Aharon were barred from entering the Land.
Also prominent in Chukat is the copper serpent episode at Bamidbar 21:8, where God instructs Moshe to mount a seraph figure so that anyone bitten who looks at it shall recover, and the song of the well at Bamidbar 21:17, where Israel sings 'Spring up, O well,' with Miriam's death recorded tersely at Bamidbar 20:1 — a juxtaposition Rashi uses to derive that just as Moshe and Aharon had water through Miriam's merit, her passing triggered the well's disappearance.
In Balak, Rashi's most celebrated passages cluster around Bila'am's talking donkey: God opens the jenny's mouth at Bamidbar 22:28 and the donkey confronts Bila'am with his own behavior at Bamidbar 22:30, episodes that Rashi treats as a deliberate divine rebuke of the would-be curser.
Bila'am's oracles also generate well-known Rashis: the theological declaration that 'God is not human to be capricious' at Bamidbar 23:19, and the exclamation 'How fair are your tents, O Jacob' at Bamidbar 24:5, which Rashi connects to the modestly arranged entrances of the Israelite encampment.