Why split it to nationalities?
It's an impossibility to gauge humour. Humour is of course subjective, but we also have the problem that a lot of humour is based around wordplay, and that doesn't translate very well. There are some very funny people for whom English isn't their first language, and so a lot of their material would be lost on most of the forum. And then there are those who are very funny, but work in their second language; this does mean that the turnover of new material is sometimes slower (Henning Wehn is one that springs to mind here).
You could therefore argue that, by giving us the great silent comedians, that the Americans deserve first place because the likes of Buster Keaton and Charlie Chaplin are great comic actors whose work has to transcend national and cultural boundaries.
And then, is nationality really the defining factor or is it a cultural, or class thing? Much of Britain's best-loved comedy was created by upper- and upper-middle class university theatrical society types - Blackadder, Python, Fry and Laurie etc. On the cultural side of things, then there has to be a very strong case for the Jews being the funniest people - they cover all bases of comedy and across national boundaries, from the timeless brilliance of the Marx Brothers to the time-honoured style of Jerry Lewis, from British wind-up merchant Simon Amstell to the neurotic romanticism of Woody Allen, from the aggressive ranting of Lenny Bruce to the social ridicule of Sacha Baron Cohen, from the craziness of Harry Shearer to the legendary work of Jerry Seinfeld, from the brilliant observances of Phil Silvers to the classic work of Peter Sellers, from the influential working-class ethic of Roseanne Barr to the timeless cartoon comedy of Mel Blanc, from the Hollywood nonsense of Jack Black to the self-conscious paranoia of Larry David, from the legendary farce of Mel Brooks to the puerile jokes of Andrew Dice Clay, from the cinematic brilliance of Gene Wilder to the simplicity of the Three Stooges, from the satirical directness of Al Franken to the surreal mockery of Rebecca Front, from the child-like Pee-Wee Herman to the left-field genius of Andy Kaufman, from the legendary Russian satirist and impressionist Gennady Khazanov to the madcap musical mind of Tom Lehrer, from the ridiculous mind of Patrick Marber to the none-more-Jewish act of Jackie Mason, from the timeless Soviet stand-up Arkady Raizin to the flash-in-the-pan childishness of Sarah Silverman, from the knowingly offensive Bob Saget to the quick mind of Joan Rivers, from the mindless idiocy of Adam Sandler to the very British physical humour of David Schneider, from the compare-and-contrast exercises of Yakov Smirnoff to the shock-and-awe of Howard Stern, from the popular satire of Jon Stewart to the ageless one-liners of Henny Youngman.
Yes, it was quite a bit of work going through all these. I saw a statistic - that was unverified - that 2.5% of the US's population is Jewish, but 70% of its working comedians are.