Kraftwerk! (The world may have finally caught up to them though, after 30+ years of being ahead of their time.)
Melvins (#1 in heavy rock)
Pavement (so many quirky indie rockers out there to this day! Of course there's more than a little of The Fall in their sound . . . )
Metallica (they've probably shifted the most units around the globe, took hardcore metal from niche to mainstream)
Michael Jackson (key influence on Britney, Justin, Backstreet Boys, etc. -- for better or worse, he is/was "the king of pop.")
Madonna too? ("girl power", "riot grrrrl" and all that plus dance music in general. Though William Orbit gets a lot of credit for that part.)
I don't see Radiohead as that big a deal, but I suppose U2 has a lot to answer for.
Hip-hop has obviously been huge for the last 25 years, but the trends there change so quickly there's no "one" most influential artist (LL Cool J has lasted longer than anyone else I suppose.)