Here is a list of designers that create some of the best wordpress themes on the net. The list is in no particular order and does not cover designers that port CSS or XHTML themes.

http://www.jauhari.net | Themes created: 21 | Theme highlights: pitr – MyApril – jblock

http://www.wpdesigner.com | Themes created: 40 | Theme highlights: Quadruple Blue – Peaceful Rush – See you at the beach

http://www.briangardner.com | Themes created: 9 | Theme highlights: Activate – Vertigo – Dropshadow

http://www.thoughtmechanics.com | Themes created: 9 | Theme highlights: Benevolence – Bionic Jive – Squible (Note: they seem to be in the midst of a redesign or something.)

There are other great authors of wordpress themes but above are in my humble opinion the best for style, simplicity, and overall design.