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The hardest and finest stones I have seen so far are from Maksim.

Takeshi does not hone razors and does not know which stones are good and which are not so good. Maksim tests all stones on razors so it is a very safe buy.
As far as the small cheap stones from...

nat stones are best used to bring the contrast between jigane and hagane.
For regular use no need to go above 5k.

I'd be cautious what Mark says, only because he is not honing razors in the numbers we do. That stone is in the 6-10k range and is a prepolisher.