I'm going to be that guy and say that I miss SMF 1.1's default theme before curves and gradients were everywhere. It was simple, it was elegant, it was classy - and yes, it was a product of its time, but I still miss it.
https://smfthemedemos.levertine.org/index.php?media/item/core.3/ in case anyone doesn't remember/hasn't seen it. I also miss the default theme Wedge had at once stage during its development but since that wasn't released and the theme changed afterwards, it's hard to say. Lost in time, like tears in rain.
Of the current iteration of platforms, I'm kinda eh about all of them. I think XF 1.x was more opinionated and had a point of view in a way 2.x's doesn't and that doesn't do it favours. IPB is *fine* I guess but it doesn't wow me. They're all functional but.... that's it really.