Right, this may sound harsh here, but the rules are:
1) ONLY 1 song allowed
2) don't post saying "I don't know, it's too hard" and then leaving it at that.
3) Say a little about the song, maybe explain why it's your favourite 
4) not a rule, but let's see how varied things get
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Right, after MUCH time and thought, i've finally decided on my most favourite song of all time. I was always under the impression that it would be an Ayu song, no contest, but i was mistaken.
My favourite songs, that i don't think i'll ever get bored of is...
Koda Kumi - 1000 no Kotoba
I seriously love this song, and hearing the (almost) 7 minute orchestral version has just made my mind up. It's perfect imo, amazing vocals, great melody, powerful and touching all in one.
Koda, I thank you for this song