I've used Headway and Thesis for clients so far, considering using OptimizePress (unfortunately I have clients setup using the Multisite function which isnt compatible with OptimizePress).
Of course there's always the gazillions of premium themes for specific niches as well...but I'm looking more at the themes that can be customized for each client.
I am considering this theme however:
WordPress for Business Websites - The SmallBiz Wordpress Theme - A Complete Web Presence for Your Business
What about you guys?