I regard @Kim as the best designer on xenforo. She creates her own designs icons from scratch and you know what you are getting is going to be unique and a one off design when it comes to imagery. But I don't think she is taking on custom jobs.
Forsaken, Russ audentio, CyberAp, John are excellent designers to and a few others who I can't remember off the top of my head.
It's surely got to depend on personal preference, and the individual site requirements. The only way you would get an (opinion based) idea on who is 'best' would be to get everyone to design a theme for a given set of requirements.
You will not get an objective opinion about who is the best, because your opinions might be different from everyone else.
You might start gathering a list of designers and then you can ask them to show some of their work. After you review each and everyone of them, you will have a more personal and useful opinion about who is the best designer for your personal needs.
This is my main concern, I have a guy who maybe able to do the custom skin I want but the last thing I want is a skin that will slow the Forum down, as I am moving from vB to xenforo for the speed of it all.
So a good skin guy that people know that won't slow down the Forum loading
Exactly... for one forum there wasn't anything out that would fit so I just modified the default skin for what I wanted it to look like (it still needs some work). For another site (an Apple Forum) I went with xenFoogle Pro by pixelExit (@Russ & @Qwk86gn) modified to fit better to the Apple environment. The third forum is running Casual (free skin also by pixelExit).