The answer is: most famous feature.
An adverbial phrase can contain one or more words which function as an adverb, being the adverb the head word of the phrase. Thus, they modify an adjective, a verb or another adverb. In the example sentence, the word most is the adverb, which means to the greatest extent.
The phrase "its ring system" is a noun phrase, "of ice particles" is a prepositional phrase, and "that is made" is a defining relative clause.