I haven't seen as many Elphabas as most of the folks here, but Willemijn Verkaik was my first and thus far my favorite. To me, her Elphaba just looks dangerous, full of smoldering passion, controlled yet given to sudden outbursts, and a woman you don't want to cross - a clear threat to the established order. I've always preferred Elphaba to be less poor, misunderstood girl and more emotional powder keg with a dark side that given sufficient motivation and resources would burn Oz to ground.
I love Jackie Burns' "No Good Deed" on You Tube. She looks like she is having an absolute blast being the bad guy and is just so deliciously hammy. I wish I had seen her live.