The people I have seen tackle the role of Elphaba are:
Shoshana Bean (x2)
Carmen Cusack (x2)
I have unbelieveable respect for anyone who can play this role. The role is probably the most physically/vocally/emotionally draining roles on Broadway right now, or ever for that matter. Elphaba belts into the stratosphere NON STOP throughout the show, and each of the actresses I have seen have made it look so easy. The two Elphabas that top out my list of Elphabas I have seen live are Christine Dwyer and Carmen Cusack. If you needed one answer, I would say Carmen.
Carmen's voice is completely unique in this role. If you have seen her, you know what I mean. If you have not, then you don't. If you go on youtube and search for her Wicked videos, you will probably not be very impressed, because videos do not capture the power in her voice. Her voice fills the theatre, and pins you to your seat. She has a very triumphant and loud sounding voice, which is perfect for big numbers such as Defying Gravity or No Good Deed, but it is also soft and pretty when it needs to be for numbers like I'm Not that Girl and For Good, and she makes the perfect transition between the two for songs like As Long as You're Mine and The Wizard and I She acted the role very very well, and was funny, and layered, and you can't help but scream for her when she comes out for curtain call :) Her chemistry with Katie and Meggie was exeptional.
Christine Dwyer is amazing for similar and different reasons. She is much more powerhouse, and she has a nasal sounding speaking voice (in an AWESOME way). She was very sarcastic and had awesome Chemistry with Natalie Daradich.
Donna Vivino is my other favorite. I have not seen her, but all I have seen online (which is A LOT) is amazing. She has the strongest voice I have ever heard in my life, and she sings the score with ridiculous ease. She acts the role very well, and you can tell she loves doing what she does. I hope to see her move to lead on Broadway after Jackie Burns finishes her contract! :) She led the tour for 2 years, and is certainly more than capable of leading the Broadway cast as lead Elphie! :)