I own Rin so she's the "obvious" vote. Yet her vowels can sound disgusting at times.
Megpoid is good (not great). Luka and Yukari are bland. Miku's English can sound OK if tweaked some (using the CHARACTER parameter. As for RUBY, I don't like her for the same reason I dislike Dex. Sounds "too American."
(But what about Amy, Kaori, Prima, or Avanna?)