James Macpherson (1736-96) (portrait, very fine!, by George Romney*) the greatest British author you've never heard of...
At Albion Awakening; I argue that the 'Ossian' poems of James Macpherson, beginning with the publication of 'Fragments of Ancient Poetry' in 1760, is of great importance to our literature and thinking; and this achievement ought to be more widely known and respected.
*Note: I am quite a big fan of what I term The Four Rs of 18th century portraiture: Reynolds, Raeburn, Ramsay and Romney. A few years ago I saw a big exhibition of Romney portraits at a stately home... he did a lot of really good work.