DISTANCE, because it's one of the songs I heard from Hikki (others were Hikari and Kettobase). I was really hooked into this song after I first heard it and it really started my Hikki fandom.
I also love all of the other songs, yet I think First Love is the second because that really brings me memories.
And Deep River is very beautiful too, I love singing it myself too. <3 Exodus and HEART STATION are also nice, yet the latter is better. But DISTANCE is absolutely the best of these.