http://www.ubuntu.com/certification/

In my experience Dells usually work well and if you're willing to spend the money they have some pretty decent end developer laptops that come with ubuntu (xps I think).

Edit: seeing as Ubuntu 14.04 has only been out for a couple weeks the links provided may not be as relevant because they're so far only certified to run on 12.04, but it still gives you some inference as to what manufactures have the best support.