The best replacement for windows server 2012 is ubuntu server.

The best alternatives to windows server 2012 are rated as following:
★★★★★  ubuntu server
★★★★★  freenas
★★★  datacenter
★  windows home server 2011
★  windows server 2008
★  home server 2011
  2008 r2

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Speaking about windows server 2012, one probably shouldn't forget about sharepoint 2010.

Other search terms related to windows server 2012 and their ratings are:
★★★★★ sharepoint 2010
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windows server 2012
ubuntu server
freenas