Rashi: It is Har ha'Bayis.

Radak (Melachim II, 19:23): It is Eretz Yisrael, which is higher than all other lands.

Radak (Melachim II, 19:23, citing Targum Yonasan): [I ascended to seize] walled cities.

Malbim: Hashem's dwelling is surrounded by mountains.

Rashi: It is the Beis ha'Mikdash [Rashi Melachim II, 19:23, from Sifri va'Eschanan 28 - which is Malbin (whitens) sins].

Radak (Melachim II, 19:23): It is Eretz Yisrael. It is called Levanon, a tree forest in Eretz Yisrael. Gedolei Yisrael, i.e. the officers and Giborim, are compared to pines and cedars. He calls the land Levanon; i.e. I conquered the land, and now I come to Yerushalayim - "Melon Kitzo."

Malbim: They are forests on the mountains around Hashem's dwelling.

Rashi: I will not return [to my land] until I destroy [the Beis ha'Mikdash].

Malbim: Sancheriv already began cutting trees in the forests on the mountains around Hashem's dwelling, to make a path for his troops.

Rashi: It is the Beis ha'Mikdash, which is Melaben (whitens) sins.

Radak (Melachim II, 19:23): It is Yerushalayim. It is called Melon due to the Beis ha'Mikdash in it, which is the dwelling of Hashem and His lodging place. It says Kitzo, for it is as if Yerushalayim is at the end of Eretz Yisrael, for [Sancheriv] conquered all of Eretz Yisrael except for Yerushalayim. Melon Kitzo is like "u'Va Ad Kitzo v'Ein Ozer Lo" (Daniel 11:45).

Rashi: It is the best and strongest part.

Radak (Melachim II, 19:23): It hints to Yerushalayim, the best, most honored part of Eretz Yisrael. [Crops] are seeded in Ya'ar Karmelo, and tree fruits grow. It is better than a tree forest. Ya'ar ha'Levanon are barren trees.

Malbim: It is the choice forest that Sancheriv already conquered ("u'Varad b'Redes ha'Ya'ar" - 32:19), after which it was easy to conquer Yerushalayim.

Radak (Melachim II, 19:23): These mean the same. It says Merom Kitzo because the Beis ha'Mikdash is in a high place, on Har ha'Moriyah.

Radak (Melachim II, 19:23, citing Sanhedrin 94b): Sancheriv said that first he will destroy His dwelling below, and later His dwelling above.