I really do like the Czar theme, and I'm not just saying that because it's what I'm used to. It's a really readable board style. If you like, I can fiddle with the images and .css to recreate the Czar Green theme out of the Czar Orange. Or hell, I could even do Czar Blue or Czar Charcoal or Czar Periwinkle if anyone wants one of those (not Czar Periwinkle).
But as far as existing themes go, I like these two:
http://demo.phpbb3styles.net/Avalon
http://demo.phpbb3styles.net/ktukblack
Because they're clean and readable. After some thought, I'm not really digging the color highlighting on the posts with the current theme, and the icons are rather ambiguous and confusing. I mean, what does this mean: <img src="http://www.scirra.com/phpBB3/styles/saphic/imageset/topic_read_hot_mine.gif">. Haven't got a clue, but it sure is big and blinky.