I agree with all the ones mentioned, but I really think Daphne has SUCH an expressive voice. On the RENT OBCR, it's really moving and really obvious as to what emotions she is portraying. I definitely agree about Adam!
"Blow out the candles, Robert, and make a wish. Want something. Want something."
Voting for Adam, obviously. Not as THE most, because that's probably silly, but as one of the most. He does his best acting through music, and like Lael said, the amount of emotion that goes into his vocals and his body language when he sings is... well, ridiculous.
I think a few of the people listed here do one or two emotions- say, angry and angsty- really well, but not much else. But it's all subjective, so whatever. :)
A few who came to mind for me: Julie Andrews Barbara Cook Rachel York Bernadette Peters Elaine Stritch Joanna Gleason