I don't have a favorite song of all time. I don't look at songs that way. I have a favorite song per band, if that counts. I suppose technically... "Princess" could be considered the ultimate because it's my favorite song by my favorite band but... Well, I have a list for you:
Versailles: "Princess"
Matenrou Opera: "Alkaloid Showcase"
Acid Black Cherry: "Black Cherry"
D=OUT: "Shangri- La"
heidi.: "Shounen"
abingdon boys school: "Howling"
Hizaki Grace Project: "Race Wish"
L'arc~en~Ciel: "Pretty Girl"
LM.C: "EDO FUNK"
Mix Speaker's, Inc.: "Romeo no Melody"
GACKT: "Vanilla"
I wasn't sure if this was the right cover... Haruko help!
D'espairsRay: "Love is Dead"
BUCK-TICK: "Dress" (in this case, I like abingdon's cover more melodically, but what classifies it as a favorite are the lyrics. I love the tormented, delirious, contemplative nature of the song.)
Beyond that, I listen to a lot of random singles by random artists, but not enough to keep favorites. I even have a few JPop songs. It's a pretty relaxed list.
Day 6: Your favourite J-Rock video.
Day 7: A J-Rock song that makes you cry.
Day 8: A J-Rock song you know all the words to.
Day 9: Your favorite J-Rock performance.
Day 10: How you first got into J-Rock.
Day 11: The very first J-Rock song you ever heard
Day 12: Your favourite J-Rock singer.
Day 13: Your favourite J-Rock drummer.
Day 14: Your favourite J-Rock guitarist.
Day 15: Your favourite J-Rock bassist.
Day 16: Your favorite J-Rock lyric (and a translation).
Day 17: Your ultimate J-Rock idol.
Day 18: Your favourite J-Rock cover song.
Day 19: Best live J-Rock group.
Day 20: Your favorite picture of your J-Rock idol.
Day 21: Your favourite piece of J-Rock merchendise.
Day 22: Your favorite picture of your favorite J-Rock group.
Day 23: Your favourite J-Rock tour.
Day 24: A J-Rock song that makes you happy.
Day 25: A J-Rock group that you have seen live.
Day 26: Your favourite interview with a J-Rock group.
Day 27: A J-Rock member that you would like to meet.
Day 28: A J-Rock group you wish would get back together.
Day 29: A J-Rock song you never get tired of.
Day 30: Your favourite J-Rock album.