I'm glad that Voyage is considered to be one of her biggest hits because it's so good. It's one of the first Ayu songs I heard somewhere in 2005 or 2006, I'm not sure anymore.
I also think that the songs she usually sings on her concerts (Boys & Girls and others) are also her hits. I also think that No way to say and A Song for xx are hits since Ayu seems to sing the latter quite a lot imo (which is good since I love the song personally a lot).