Neodymium magnets are probably the strongest but also rather expensive compared to rare earth magnets.
I think you could embed the magnets in the wood block (the shiny coated ones could be left exposed) and if you want to hide them cover them with a thin veneer. A pair of 1" rare earth magnets positioned vertically a few inches apart to each other would probably hold the biggest knife. There are other shapes if you search, such as cylinders that could join end to end to form a long rod shape horizontally across the top of the holder. 
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