I currently use Jumpstart for almost all of my WordPress sites. It features a simple, responsive default look. It comes with a flexible HTML5 markup structure, a full-range of user features, and all the actions and filters you’ll need to fully customize it from a Child theme.
I particularly like the slider interface, very user friendly. The documentation is outstanding and if you don’t get an answer there the support forum has you covered.
This is a link to the Jumpstart WordPress Theme.
Jumpstart is very “vanilla”, if you want something a liittle more “finished” out of the box try one of the other Theme Blvd Themes, built on the same great framework.