I don't think you have to look much further than Toby Jepson of the now defunct Little Angels to find one of the best most underrated British songwriters ever.
For content and lyrical style he wrote some of the cleverest and thoughtful songs I've ever heard. I don't know why the band finished because they could and probably should have been one of the biggest bands in the world. Oh well, c'est la vie.
Top Amrican songwriter, well I don't know but Bruce Springsteen has to be up there, especially the earlier stuff.