Lots and LOTS of Team OS works (Hitoshizuku-P and Yama). I can't get over how these guys write Vocaloid songs, and how they use the 'loids.
They practically sound like real people. All the harmonies and layers they include are just gorgeous. Right now, the top of my list is The Butterfly, the Flower, and the Spider, and Genealogy of Red, Black, and White. They're both such tragic songs, but with fantastic tuning and great instrumentals.
One of these days, I'll sing both of them.
EDIT:
Why the heck does poor Rin keep getting raped in all of their songs, this is not okay with me. Someone really needs to get Len a restraining order, or at the least, keep Rin indoors at all times. According to Team OS, she's way too beautiful for her own good.