The best replacement for icebreaker is smartwool.

The best alternatives to icebreaker are rated as following:
★★★★★  smartwool
★★★★  ibex
★★★★  patagonia
★★  helly hansen
★★  minus 33
★  exofficio
★  energizer
★  redram
★  ice breaker
★  macpac merino
  macpac
  kathmandu
  woolpower
  capilene
  rapha
  pherlure

Your alternative for icebreaker is not listed? Suggest!
Speaking about icebreaker, one probably shouldn't forget about bj cole.

Other search terms related to icebreaker and their ratings are:
★★★★★ bj cole
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Google Trends

Scale is based on the average worldwide traffic of icebreaker in all years. Click on graph for more.
icebreaker
smartwool
ibex