I always liked the simpler designs of the kids in early Homestuck, particularly I think Jade fits the best in that early style and her witchy aesthetic when she ascends and goes Dogtier is also pretty cool. If I had to pick a beta kid who's like "the best dressed" it would be her.

And for what it's worth those simple Godtier duds are pretty neatly designed for how little effort was put into them lol. I think the blue pajamas John wears are pretty iconic, I can't really imagine him wearing anything else (unless you count the green ecto-suit he wore that outfit also sticks into my head a lot) And even though Dave wore a lot of suits and stuff, I feel like his coolest outfit was just his Godtier dudes when he was chilling on the meteor. His cowl is kind of lame looking, so I can see why he doesn't wear it that often even though I think it's supposed to make him look more "Knightly" lol

Rose has a lot of good looks and I can't pick just one so I'll just say she and Jade were also the best dressed.