Re:UX leaders?

Flow Interactive in London
Adaptive Path in San Francisco

I am sure there are more...but I can't think of any.  Most agencies  
are either HCI/Usability houses or classic design agencies with some  
UX people attached for good measure...

Look for companies with case studies which show genuine measurable  
results and good design.  Also look for processes in which research  
and design are genuinely integrated, teams are multi-dicsiplinary,  
and design and research are done in iterative cycles from design  
strategy all the way through build.

good luck