Adam Smith, Friedrich von Hayek, and John Maynard Keynes. Whilst I cannot agree with much of what Keynes wrote (full employment, fixed exchange rates, inevitability of depression, demand impact of inflation, etc), his work shaped the world in which we live like no economist since Ricardo.
BTW, I would advocate the addition of Sir Isaac Newton (the first person to apply scientific methods to the area of finance, and also inventor of the bimetallic standard), but I fear that he would receive lots of votes based on his scientific work.
Whilst I cannot advise on that addition, I can advise on deletions. Marx, Luxemburg, Malthus, and Aquinas, by any definition, were not economists. The most flattering way to describe Marx and Luxemburg is 'philosopher', although they were far from philosophers in the usual vein. Malthus was a demographer, whose bleak outlook for humanity was based on real (bad) economists' work. Aquinas was a theologian, and didn't come close to economics (nobody did, until Newton). I suppose that Aquinas is the token Italian.