The best replacement for d600 is d800.

The best alternatives to d600 are rated as following:
★★★★★  d800
★★★★★  d610
★★★★  d7100
★★★  d700
★★★  d7000
★★★  6d
★★  canon 6d
★★  5d mark ii
★  d4
★  k750i
★  a7
★  5d mark iii
★  d500
★  d3200
★  w800i
★  d90
★  d5100
★  d5200
  d600e
  d800e
  d300
  nikon d7100
  nikon d800
  v3i
  w800
  70d
  d800 review
  7d
  df
  nikon d700
  d900
  used d700
  canon 5d mark iii
  d800 video
  nokia 6270
  600m
  d800 reviews
  t809
  n70
  sony ericsson w800i
  d820
  dell d610
  nikon d7000
  omd
  d3
  t42

Your alternative for d600 is not listed? Suggest!
Speaking about d600, one probably shouldn't forget about dx lens.

Other search terms related to d600 and their ratings are:
★★★★★ dx lens
★★★ dx lenses
★★ 24-70
★ 16-35
★ kit lens
★ 28-300
You have in mind a companion for d600 and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of d600 in all years. Click on graph for more.
d600
d800
d610