Let me see...
Original: Youyoumu ~ Snow or Cherry Petal
Yes, the first Touhou track I heard was the menu theme of PCB. It's why it holds a special place in my heart, no matter how annoyed I get at Yuyuko.
Instrumental Remix: Night of Nights (Flowering Night)
Gosh, this is going back to when I was crazy over Lucky Star & Suzumiya. I heard a Lucky Star mash-up with a 'really catchy track' and thought of finding it on its own. And so, beatMario takes that honour.
Vocal Arrange: Cirno's Perfect Maths Class
At the same time, I was known throughout my old anime club for knowing anime related dances. So as I decided to try and find other ones to learn, I came across a bunch of guys dancing in the snow; one of which was 'L' from Death Note. And that led to IOSYS being my first vocal song.
Pretty mixed bag there. ^^