Which two elements have chemical properties that are most similar?

This is not an entirely objective question, but many would consider any two lanthanide metals to meet this test. The two heaviest two elements in any wide-form periodic table column would also be good candidates, as would zirconium and hafnium, because the lanthanide contraction result in their being almost the same size even though one of them is from a higher period than the other. An objective answer would require specifying some specific chemical property with the least quantitative difference between two elements.