The best replacement for canon 6d is nikon d600.

The best alternatives to canon 6d are rated as following:
★★★★★  nikon d600
★★★★★  5d mark ii
★★★★★  70d
★★★★★  5d mark iii
★★★★  nikon d610
★★★  sony a7
★★★  nikon d7100
★★★  nikon d800
★  sony a7r
★  mark ii
★  d600
★  nikon d7000
  fuji x-t1
  canon 5d mark iii
  nikon d700
  fuji x-e2
  canon 70d
  canon 5d mark ii
  5d mark 3
  5d mark 2

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Speaking about canon 6d, one probably shouldn't forget about 35mm 1.4.

Other search terms related to canon 6d and their ratings are:
★★★★★ 35mm 1.4
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canon 6d
nikon d600
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