The best replacement for pixelmator is photoshop.

The best alternatives to pixelmator are rated as following:
★★★★★  photoshop
★★★★  gimp
★★★★  aperture
★★★★  acorn
★★★  photoshop elements
★  sketchbook pro
★  sketch
★  idraw
★  lightroom
★  photoshop cc
  snapheal
  photoshop elements 9
  iphoto
  graphicconverter
  sketchbook
  aperture 3
  photoshop elements 10
  intensify
  acorn 4

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

Other search terms related to pixelmator and their ratings are:
★★★★★ aperture
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Google Trends

Scale is based on the average worldwide traffic of pixelmator in all years. Click on graph for more.
pixelmator
photoshop
gimp