Kagamine Rin is my favorite Vocaloid. It'll probably changed to Pico once I hear more of his songs.
I like Rin, but sometimes it seems like she can't sound very good unless edited some xD;; A lot of times, her voice just sounds ick when it's coming right out of the program. She's just a little harder to use I guess, but there are tons of songs where she sounds awesome o.o
I like Piko's voice too, but I've mostly heard crappy covers of almost every song ever :/ I hope he gets more originals.