I think all the designs are really wonderful, but I'd pick Young as well. Loquasto's are lovely, but he had the blueprint of the original production to work from, and Zuber's designs are period clothing with inspiration from the actual two ladies, sumptuous though those designs are. Young had to create a visual language for Great Comet from scratch to match the anachronistic nature of the storytelling, and I think she did so beautifully. Natasha's white coat, in particular, is a stunning, instantly iconic design that's up there with Dolly's red dress. Based on pure inventiveness and originality, she should take it.
I am a firm believer in serendipity- all the random pieces coming together in one wonderful moment, when suddenly you see what their purpose was all along.