I'm going to torment y'all with more Obey Me posting, here's what sorts of dresses I think the boys would pick for MC ( regardless of gender)
Lucifer: He can act like he's not as horny as his brothers, but I think he would imagine something super form fitting. Classy, true, but form fitting and probably black or blue or red. Feather details are a bonus.
Mammon: Thinks MC is so, SO hot, so something short and revealing, decorated in glitter and gold. Obviously something in black and gold. That being said he'd also all but faint to see them in something white and shining, loosely hanging over them like some old depictions of the gods.
Leviathan: Also horny, but also a nerd. Something with fantasy elements, and definitely at least one leg slit. He also likes oceanic colors, or orange to match him of course. Tbh his dream dress for MC would make them look like a mermaid.
Asmodeous: He's also horny (noticing a trend) but he ALSO loves fashion. I think his ideal dress would look like really really fancy lingerie. Definitely heavy on the lace, and definitely something in pink or white.
Satan: A hopeless romantic who loves fairytales? He absolutely would choose some kind of princess looking gown, with a structured bodice and fluffy skirt. Naturally he'd like something in green, or maybe purple.
Beelzebub: I feel like he doesn't think about this kind of stuff much, but if pressed he'd probably pick something short and cute. Or something that made MC's butt look good, he's totally an ass man. I feel like he'd like the color red too, or black.
Belphegor: Now he would want something that's soft and nice to touch, so probably something silk. Naturally he'd like something in purple or deep blue, super dreamy nighttime colors. It also doesn't matter how small your boobs are or if you even have them, this guy wants to see and rest his head on your cleavage. (Brat)
Diavolo: Something luxurious and regal, he wants to show MC the extent of what he can offer them. Also something white and gold, it's gonna end up looking like a wedding dress. He does not care, in fact that's a bonus. Something that glitters and makes them look even more divine. (He low-key wants to make them look more gorgeous than the angels as a flex)
Barbatos: Something slinky and formal. Something that matches the turquoise of his tail. Where Mammon would drape MC in gold, Barbatos goes for jewels. On MC's neck, chest, in their hair. He wants to see them glimmer.
Simeon: Something sweet and power blue. He prefers fabrics that drape and flow, loosely hightlighting their form. He'd never admit it but he also has a thing for dresses that are a bit transparent...
Solomon: While this asshole would probably say he'd prefer them nude, there is a legitimate answer. He would like a dress that looks like the night sky, black with glittering silver stones. Something that makes MC look as magical as he thinks they are. That being said he also wants a really REALLY high leg slit with a garter. He'd have a conniption.