The best replacement for nikon d300 is d90.

The best alternatives to nikon d300 are rated as following:
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★★★★★  canon 40d
★★★  d300s
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★★★  canon 5d
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★★  nikon d90
★  d200
★  canon eos 40d
★  d2x
  40d
  canon d40
  nikon d300s
  pentax k20d
  nikon d80
  nikon d700
  nikon d200
  canon eos 5d
  canon d50
  d40
  nikon d3
  nikon d2x

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