Who is it? Lindy Ruff must be getting up there...
Do you mean consecutive years with one team?
Some candidates without interruption:
Jack Adams (1927-1947): http://en.wikipedia.org/wiki/Head_Coaches_of_the_Detroit_Red_Wings
Billy Reay (1963-1976/77): http://en.wikipedia.org/wiki/Head_Coaches_of_the_Chicago_Blackhawks
Toe Blake (1955-68): http://en.wikipedia.org/wiki/Head_Coaches_of_the_Canadiens
Lester Patrick (1926-39): http://en.wikipedia.org/wiki/Head_Coaches_of_the_New_York_Rangers
Al Arbour (1973-86): http://en.wikipedia.org/wiki/Head_Coaches_of_the_New_York_Islanders