Slayer and Metallica did not start metal. They brought it to new levels, sure, but they did not start metal. There is much debate over who started Metal, but I will not get into that here.
I would say the most influential music band/artist of all time would be Chuck Berry. As it was said before, he influenced The Beatles, who brought Rock and Roll to new heights. As Mike Rotch said before, all band genres (Metal/Rock/Punk/Alternative/etc) wer spawned from Rock and Roll, and as hose genres, along with Rock and Roll, have been some of the most popular for years now, I would say that the Father of Rock and Roll deserves this title.