Affirmative: くだもの　がすきです　I like fruits. くだもの　だいすきです I like fruits very much. Negative くだもの　は　すきじやないです I don't like fruits. くだもの　あまりすきじやないです。I don't like fruits but I don't hate fruits. あまり〜じゃない It's use for Negative sentences.　あまりmean "a little".

The differences in these sentences are both a matter of degree. Regarding your fruit appreciation sentences, the first sentence says you like fruit, while the second with 大好きis YOU REALLY LIKE FRUIT! Regarding your dislike of fruit, the first sentence says you do not like fruit, while the second sentence is tempered somewhat and might be translated as “don’t like fruit that much”.