Teto, of course. I was derping on a forum and discovered 'Zutto Teto no Turn.' That also happened to be the same day I found out about Vocaloid.
For Overseas, I'm pretty sure it was Utaune Nami (she is technically Overseas, right? Iunno. Correct me if I'm wrong) with her WAVEFILE cover.
When I first started learning more about UTAU I thought it was super amazing that you could 'make your own Vocaloid.' xD