Hi, we have the formal and informal form and in formal one, it's わたしはにほんがだいすきです(私は日本が大好きです)。and in informal one, わたしはにほんがだいすき(私は日本が大好き/大好きだ)。You can omit 私は when everyone knows the subject, and between close people, in colloquial, you can say 日本大好き!I hope this was helpful.