Tohoshinki - Gosenshi
One of Yoochun's favorite song (I love it too!) and he even said, whenever he heard this song, it reminds him of Junsu's Smiling Face *cough* yoosu *cough*
P.S. remeber to listen till' the end, there's a surprise!
*Spoilers...* Don't read more... if You Don't want it.
Credits: DBSKnights + delusion + saki3649