The best replacement for d300 is d90.

The best alternatives to d300 are rated as following:
★★★★★  d90
★★★★  d300s
★★★  40d
★★★  canon 40d
★★★  d700
★★★  d7000
★★  5d
★★  d3
★★  d80
★★  d2x
★★  d7100
★  d200
★  50d
★  d5100
★  d3200
★  d600
★  nikon d90
★  canon 50d
  d70
  d500
  d800
  d40
  iva d310
  d2xs
  d3100
  nikon d7000
  d2h
  k20d
  d60
  nikon d300s
  d5000
  nikon d200
  nikon d700
  d3000
  canon eos 40d

Your alternative for d300 is not listed? Suggest!
Speaking about d300, one probably shouldn't forget about 24-70.

Other search terms related to d300 and their ratings are:
★★★★★ 24-70
★★★★ mb d10
★★ 50mm
★ 35mm
★ sb800
★ 24 70
mb-d10
You have in mind a companion for d300 and it is not yet here? Propose!



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d300
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