The best replacement for ess is oakley.

The best alternatives to ess are rated as following:
★★★★★  oakley
★★★★  revision
★★★  bss
★★★  wiley x
★★  dss
★★  burr brown
★★  rstudio
★★  dinan
★★  css
★  eis
★  eav
★  flakjak
  lsp
  hass
  usma
  vf
  hpf
  mca

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Speaking about ess, one probably shouldn't forget about emacs.

Other search terms related to ess and their ratings are:
★★★★★ emacs
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Google Trends

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ess
oakley
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