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Metz makes very good flashguns, that will work with the Nikon Speedlights if you acquire them in the future. I bought my wife the Metz flash that is roughly equivalent to the Nikon SB-700, and it works well on her D7000. Also, when used as a remote (slave) flash, our SU-800 will fire it reliably.
On my Canon cameras, I have been pleased with the performance of my Nissin Di-866 II Professional. This one is available in a version for Nikons.
Really, however, we both favor our SB-800 Speedlights, which I acquired as pre-owned merchandise, at some saving in cost compared to the then-current SB-900.
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