Malbim (8, 14): A king should be the most worthy. The masses do not select him; the leaders and sages choose. Initially, the choice (fruit bearing) trees offered kingship to the olive and fig trees and grape-vine. Now, the masses, i.e. the majority of trees, which are barren, offered it to the Atad, the lowliest. It has no benefit; its only advantage over the others is that one cannot approach it due to its painful thorns - "u'Vliya'al k'Kotz Munad Kulaham" (Shmuel 2 23:6). I.e. Avimelech's only attribute is that he is full of murder and spears.

Torah ha'Temimah (9:8, 6, citing Gevi'i Gevi'a ha'Kesef): When they went to the each of the three esteemed trees, the other two did not go, lest they tell them 'you should rule, for also you are esteemed!'

Rashi: It is a rose tree.

Ralbag: It is a kind of thorn ¹ , The Targum of (Bereishis 3:18) "Kotz v'Dardar" is v'Atedin.

Bereishis Rabsi (20:15) It represents Avimelech, who had no good deeds. It had no reason to decline!

Radak (9): They crowned the lowest to be king, i.e. Avimelech. A king should guard the people from harm. Thorns have no benefit. They do not guard people from harm. They harm people!