Best job at replacing an "Irreplaceable" front man/lead singer.?

I would have to go with Brian Johnson. Though both replacements had similar situations. Bon Scott and David Lee Roth had distinctive unique voices, Sammy Hagar replaced DLR at a time when rock and roll was transitioning in sound. 'Van Hagar', could have been, and sounded like, a complete different band than Van Halen, where as ACDC kept on rocking with the same sound. To do this with a different front man was amazing.