I personally enjoy Invision and their line of boards. Whether paid for, or free, these boards are some of the best in their line when it comes to the administration control panel and are very easy to use from a rookie to a veteran and the features are just unbeatable. The picture below is the AdminCP for InvisionFree, but I will see if I can get some IPB AdminCP boards up as well.
Besides the ACP though, the look of Invision is as professional as you need or as lax as you want. With its range of different skins, to different styles, Invision and their users have developed and are developing some of the most unique features when it comes to customization.
{Invision Power}
{Invision Free}
{InvisionFree Skin Zone} - IF support/fan forum with skin designs and tools to make your own as well as different scripts to put on your very own IF board.
Pavan.