Oh, that's difficult. That's really difficult. Hm... either Festival of Asylum (Miku Hatsune), Meltdown (Rin Kagamine) or Last Night, Good Night (Miku). It would depend a lot on my mood at the time, methinks. I'll go with the last if I have to choose.
http://www.youtube.com/watch?v=C6QsIRxtszA&fmt=18 <3