You has budget in mind?
Personally, I think everyone should have a fast 35mm or 50mm prime in their bag, but that's me... The Nikon G and D versions of these lenses are about dirt cheap and they're really, really good lenses. They're so good and so cheap you should have one of each on *principle* alone. The Sigma "Art" versions are better, at least in my experience, but they're top-notch FX lenses and the prices reflect that. If you shoot wide you might want to consider something like the Tokina ATX 12-28mm f/4 Pro.
What you should buy depends on what you need... But I'd suggest if you don't need, that you don't buy.
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