By European, I guess, you mean researchers that work in Europe.

In my case, I'm very biased because I mainly know people that work with iPS in neuroscience and most of the research comes from the USA (Studer, Jaenisch or Izpisúa for example). In Europe, for me, the main reference is Oliver Brüstle (and people from the Institute of Reconstructive Neurobiology in Bonn). They do very good research. 

Then you have other excellent researchers like Thomas Graf, Madeline Lancaster, Frederick Livesey, Magdalena Götz, Christine Mummery and Hans Schöler, among others.