The best replacement for nikon d3100 is canon t2i.

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★★★  canon t1i
★★  canon rebel t2i
★  nikon d5100
★  nikon d5000
★  t2i
★  canon rebel
★  d7000
★  nikon d3200
★  canon 550d
★  canon eos rebel t3
★  sony a3000
★  pentax k-x
★  canon 1100d
★  canon eos rebel t2i
★  canon rebel xs
★  d3000
  canon xs
  canon 600d
  sony nex 5
  canon 500d
  canon rebel t1i
  canon eos t2i
  pentax k-r
  canon xsi
  rebel t2i
  pentax kx

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Speaking about nikon d3100, one probably shouldn't forget about sigma lens.

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