Konnichiwa
So, the most common would be Kanji. >.< Kanji is a must and is very difficult to learn with so many out there. THat's why it's best to stuyd a Kanji a day. Also Hiragana is used with Kanji. Katakana is for foreign words. But Hiragana and Kanji are the most common. ^-^ But they are all common. I suggest you study Hiragana and Katakana first