@Mark Weber, maybe because the entire Clannad OST is great! haha
As for me, I have to get pumped before I workout, so as for me I start off with 2:
Zedd – The Legend of Zelda (Original Mix)
http://youtu.be/zLzaZ_HXmjw
Mord Fustang – Lick the Rainbow
http://youtu.be/iRL6kxqvhp0