The best replacement for nd filter is polarizer.

The best alternatives to nd filter are rated as following:
★★★★★  polarizer
★★★  graduated nd filter
★★★  cpl
★★★  uv filter
★★  hdr
★★  circular polarizer
★★  hss
★★  polarizing filter
★★  variable nd filter
★  high speed sync
★  exposure compensation
★  polariser
★  cpl filter
★  fader
  nd filter
  polarizer filter
  polarizing
  pl filter
  uv

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Speaking about nd filter, one probably shouldn't forget about polarizer.

Other search terms related to nd filter and their ratings are:
★★★★★ polarizer
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nd filter
polarizer
graduated nd filter