My mate is a Blackburn fan and he always says Tugay is probably the best player that has played for them but doesn't get the recognition he deserves and probably because he only came to England when he was 30.
This question really has to be someone like that, similar to Boateng, that other fans wouldn't even consider.
Most of the best players will have played for one of the top teams or played champions league football for a good chunk of their career. I do wonder why players like Southgate, Boateng and Viduka didn't play regular top level football because they seemed to me like they were good enough. Even Juninho, who is one of the best players a lot of us have ever seen, only played in the champions league when he left us to play for Celtic.