Tafsiran Ulangan 33:15: Rashi, Chizkuni, Ibn Ezra & Rabbeinu Bahya

ומראש הררי קדם AND BY THE FIRST THINGS OF THE HILLS OF קדם (taken in the sense of: the hills priority) — the meaning is: and it is blessed through the earliest ripening of the fruits, because its hills are earliest (קדם, "prior") to bring their fruits to maturity. – Another explanation: this tells us that their creation (the creation of the hills in the territory of Joseph) was prior (קדם) to that of all other hills (Sifrei Devarim 353:4).

ומראש הררי קדם; “and from the top of the ancient mountains.” This is an expression of praise, as for instance in Jeremiah 2,3: ראשית תבואתה, “the first fruit of its harvest, orAmos 6,6: וראשית שמנים, “with the choicest oils.” Moses refers to the choicest mountains that were created during the first six days of creation.

from the top Scripture employs ellipsis, meaning, “ from the sweetness of the top of the ancient mountains”.

גבעות עולם [AND THROUGH THE PRECIOUS THINGS OF] THE EVERLASTING HILLS — i.e., of hills which everlastingly (עולם) produce fruit, and do not cease to do so through restraint of rain.