The best replacement for z68 is z77.

The best alternatives to z68 are rated as following:
★★★★★  z77
★★★★  p67
★★★★  x58
★★★  x79
★★★  z77 motherboard
★★★  z77 chipset
★★★  h77
★★★  z87
★★  b75
★★  h67
★★  h61
★  wait for z77
★  p67 chipset
★  p68
★  h67 chipset
★  intel z77
  z67
  z68 express

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Speaking about z68, one probably shouldn't forget about ivy bridge.

Other search terms related to z68 and their ratings are:
★★★★★ ivy bridge
★★ 3770k
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z68
z77
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