Depends on the epoch, I guess.
Back in the late 16th Century, the Brits were certainly better, including Chris Marlow.
American authors started to assert themselves in the second half of the 19th Century, even though Edgar Allan Poe wrote before that. American originality began to manifest itself in Bret Harte and Mark Twain's writings, and in Longfellow's sporadically tedious, yet deservedly famous, poem. Jack London is certainly an American author, while his contemporary Henry James is an imitator of some pretty idiotic European standards of the time.
The father of today's science fiction, Henry Cuttner, is all but forgotten at home as well as abroad; a great American author, to be sure.
Tennessee Williams and Eugene O'Neil and Arthur Miller compare but poorly to George Bernard Shaw.
Kurt Vonnegut was way ahead of anyone writing in Great Britain at that time.
The late Douglas Adams was a stylist at least as good as Vladimir Nabokov, and a lot less bookish.
Depends.
