Rashi: It is Har Sinai, which is near Bashan in Ever ha'Yarden.

Radak: Har Sinai, even though it is in a barren desert, and it is a low mountain, it is praiseworthy, because [Hashem's] honor dwelled on it. Har Bashan is praiseworthy because it is in a fat (fertile) place and pasture. Fat cattle are called "Paros ha'Bashan" (Amos 4:1); it says "Abirei Bashan Kiteruni" (above, 22:13).

Malbim: Har Bashan is tall; it is covered with snow the entire summer, like tall and snow-capped mountains. However, when there is a blizzard, and the snow blows forcefully, it falls on camps standing on it and makes them fall into pits from which they will not rise. The verse depicts that the enemy ascended to the top of the mountain and fought with Yisrael, Hashem sent a blizzard of snow against them until it became dark. Snow covered their camp, and they perished. Perhaps this truly happened, or it is a mere metaphor for the great affliction and darkness that came on them on the mountain. A snow blizzard is awesome for people of these lands, where snow is not common. The metaphor says that the dove's wings covered the camp with snow, and it darkened. This was via Hashem.

Rashi: It is the most special mountain. "Gavnunim" is an expression of mountains, due to their Govah (height). This is like "va'Tivni Lach Gav" (Yechezkel 16:24). Everything tall is called Gav.

Radak: They are tall mountains. A tall mountain is called Gavan, because in the middle it has Gavnunis (a protrusion) from which one can see its height. Also they are praiseworthy due to their height. Har is singular, and Gavnunim is plural; each tall mountain is praiseworthy according to its height. We find like this - "v'Kibel ha'Yehudim" (Esther 9:23).

Radak: It is because it is so fat and fertile.