Most of the monsters of the week are bleh with a few standouts. If I had to pick a favorite off the top of my head, it would be Binah/Veena in terms of powers and design.
As for the best one overall, both disguised and transformed, I'd pick Murido/Dream Dolly and she also had a sick Youma reveal sequence.
Of all the early youma, though, Iguara had the best TF sequence, and I liked her monster form, even if the episode itself was needless.