The best replacement for nikon d90 is d300.

The best alternatives to nikon d90 are rated as following:
★★★★★  d300
★★★★  d7000
★★★★  canon 60d
★★★  canon 50d
★★★  d5100
★★★  d5000
★★★  canon t2i
★★  d3100
★★  d200
★★  canon 7d
★  canon
★  nikon d5000
★  d700
★  canon t1i
★  canon 40d
★  d80
★  canon t3i
★  nikon d7000
  nikon d300
  canon eos rebel t2i
  rebel t2i
  nikon d700
  canon 550d
  canon eos
  nikon d60
  canon 600d
  nikon d5100
  canon 500d
  d60
  sony a55
  nikon d300s

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Speaking about nikon d90, one probably shouldn't forget about 50mm.

Other search terms related to nikon d90 and their ratings are:
★★★★★ 50mm
★★★ kit lens
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