The best replacement for fitbit one is flex.

The best alternatives to fitbit one are rated as following:
★★★★★  flex
★★★★  ultra
★★★  jawbone up
★★★  zip
★★★  fitbit zip
★★  withings pulse
★★  fitbit flex
★★  fitbit ultra
★  jawbone
★  jawbone up24
★  fitbit force
  polar loop
  misfit shine
  striiv

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

Other search terms related to fitbit one and their ratings are:
★★★★★ myfitnesspal
★ my fitness pal
You have in mind a companion for fitbit one and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of fitbit one in all years. Click on graph for more.
fitbit one
flex
ultra