Rashi: It is strong, like "Ofel va'Vachan", "Hekimu Vachonav" (Yeshayah 32:14, 23:13).

Radak: It is a fortress - "Hekimu Vachonav" (Yeshayah 23:13). Even though you [Yirmeyah] will say harsh words to them, do not fear them. Also at the beginning of his Nevu'ah, Hashem said "Nesaticha ha'Yom l'Ir Mivtzar" (1:18).

Radak citing Targum Yonasan: It is the choice (favorite).

Malbim: Discern. Yisrael are compared to impurities in a smelting pot of silver - copper, tin, iron and lead. There were many sects; each did its own evil, but not evil of the other sects. This one did adultery, this one murdered, this one did idolatry?. Each slanders the other sects, and thinks that he is pure silver. Really, he is another dross! The Navi was sent to discern the impurities, and remove them via rebuke.

Rashi: You will rebuke them, and they will not listen.

Radak: Seek their ways ¹ . It says v'Vachanta for a similar expression to Bachon.

Malbim: Refer to 6:27:1:4.