Sir Tom Jones! Must've made Wales internationally known in a way that any amount of other worthies haven't. Bevan for his humanitarian drive, even if it's gone on to be grossly mismanaged. Vaughan Williams for the music, although perhaps not widely appreciated enough.
There must be some Welsh inventors, designers, mechanical and social engineers I can't think of right now - I'd say 'greatest' would have to be someone who's done something which has contributed to the greater good of their country, or, like Brunel, Edison or Baird, whose contributions have rippled on through time to affect us all in some way. There's got to be someone out there in Baa-Baa Land who's done that?