The best replacement for zbrush is mudbox.

The best alternatives to zbrush are rated as following:
★★★★★  mudbox
★★★★  maya
★★★★  sculptris
★★★★  blender
★★★  3ds max
★★★  modo
★★  3d coat
★  cinema 4d
★  mari
★  mudbox 2014
★  mudbox 2011
  lightwave
  silo
  maya better
  modo 701
  mudbox 2013
  body paint
  bodypaint
  substance painter
  blender on sculpting
  c4d
  hexagon
  blender sculpting
  poser
  deep paint

Your alternative for zbrush is not listed? Suggest!
Speaking about zbrush, one probably shouldn't forget about wacom.

Other search terms related to zbrush and their ratings are:
★★★★★ wacom
★★★ 3ds max
You have in mind a companion for zbrush and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of zbrush in all years. Click on graph for more.
zbrush
mudbox
maya