Here are my top 3
1. Viva Bianca
Partially since she's good at playing vain, shallow, manipulative women. Partially so Chris Hemsworth gets to work with a fellow Australian which would help the chemistry between the hero and villain, partially since she's underrated and needs more exposure.
2. Rachel McAdams
If this take on Enchantress is one where she's more icy than seductive, I'd like to see Rachel McAdams play her. Nobody plays somebody who's completely heartless yet aloof better than her but she also has a surprising range. The only reason why she's #2 instead of #1 is because she's too easy of a choice and Marvel likes unconventional casting.
3. Melanie Laurent
Inglorious Basterds proved that she could do scheming. The fact that her career hasn't taken off since then is an absolute travesty. This is one choice that's perfect yet completely out of left field that I think the fans who already have some sort of expectation of who the character is can find her redefined in the same way that Robert Downey Jr redefined Tony Stark.
Edit: Melanie Laurent pic was down.